{"id":4720,"date":"2026-02-22T21:05:03","date_gmt":"2026-02-22T15:35:03","guid":{"rendered":"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/"},"modified":"2026-02-22T21:05:03","modified_gmt":"2026-02-22T15:35:03","slug":"what-is-a-docker-image-a-complete-guide-for-beginners","status":"publish","type":"post","link":"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/","title":{"rendered":"What is a Docker Image? A Complete Guide for Beginners"},"content":{"rendered":"<p><strong>INCIDENT REPORT #9902-B: THE DEATH OF OUR REGISTRY<\/strong><\/p>\n<p><strong>Date:<\/strong> October 14, 2023<br \/>\n<strong>Duration:<\/strong> 48 Hours (and my sanity)<br \/>\n<strong>Status:<\/strong> Post-Mortem \/ Educational Intervention<br \/>\n<strong>Lead SRE:<\/strong> [REDACTED]<\/p>\n<hr \/>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_80 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<label for=\"ez-toc-cssicon-toggle-item-69d825d89e326\" class=\"ez-toc-cssicon-toggle-label\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/label><input type=\"checkbox\"  id=\"ez-toc-cssicon-toggle-item-69d825d89e326\"  aria-label=\"Toggle\" \/><nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#1_Summary_of_the_Carnage\" >1. Summary of the Carnage<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#2_The_Layered_Lie_Dissecting_the_Manifest\" >2. The Layered Lie: Dissecting the Manifest<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#3_Deconstructing_the_Manifest_Content-Addressable_Chaos\" >3. Deconstructing the Manifest: Content-Addressable Chaos<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#4_The_Overlay2_Autopsy_Where_the_Files_Actually_Live\" >4. The Overlay2 Autopsy: Where the Files Actually Live<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#5_The_%E2%80%9CLatest%E2%80%9D_Sin_and_the_Immutable_Myth\" >5. The &#8220;Latest&#8221; Sin and the Immutable Myth<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#6_The_4GB_Bloat_A_Forensic_History\" >6. The 4GB Bloat: A Forensic History<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/#Related_Articles\" >Related Articles<\/a><\/li><\/ul><\/nav><\/div>\n<h3><span class=\"ez-toc-section\" id=\"1_Summary_of_the_Carnage\"><\/span>1. Summary of the Carnage<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>At 03:14 UTC, the monitoring stack for our production cluster started screaming. Node <code>prod-worker-04<\/code>, running Docker Engine v25.0.3, reported a total disk exhaustion event on the <code>\/var\/lib\/docker<\/code> partition. Within twelve minutes, the failure cascaded. The registry, hosted on a dedicated S3-backed instance, began throwing 504 Gateway Timeouts. The reason? A junior developer\u2014who I will not name but who is currently banned from touching the CI\/CD pipeline\u2014pushed a 4.2GB image tagged as <code>latest<\/code>.<\/p>\n<p>This wasn&#8217;t just a &#8220;large file.&#8221; This was a technical crime. The image was based on a bloated Debian Bookworm environment, into which someone had decided to <code>COPY . .<\/code> their entire local development environment, including a 2GB <code>.git<\/code> folder, a <code>node_modules<\/code> directory the size of a small moon, and several unoptimized machine learning model weights. <\/p>\n<p>When the scheduler tried to pull this monstrosity across twenty nodes simultaneously, the internal network saturated. The Docker Engine, using API version 1.44, attempted to decompress these layers. The resulting IOPS spike killed the underlying EBS volumes. We didn&#8217;t just have a slow deployment; we had a total infrastructure seizure. I\u2019ve spent the last two days manually cleaning up orphaned layers and fixing corrupted overlay2 backing stores. My coffee is cold, my eyes are vibrating, and if I see another &#8220;latest&#8221; tag, I\u2019m quitting to become a carpenter.<\/p>\n<hr \/>\n<h3><span class=\"ez-toc-section\" id=\"2_The_Layered_Lie_Dissecting_the_Manifest\"><\/span>2. The Layered Lie: Dissecting the Manifest<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>To understand <strong>what is<\/strong> actually sitting on your disk, you have to stop thinking of a Docker image as a single file or a virtual machine disk. It isn&#8217;t. An image is a collection of read-only layers, orchestrated by a JSON manifest. When you run <code>docker pull<\/code>, you aren&#8217;t downloading a &#8220;program&#8221;; you are fetching a series of tarballs and a set of instructions on how to stack them.<\/p>\n<p>Let\u2019s look at the manifest of the disaster in question. This is what the Docker Engine sees before it even starts the download:<\/p>\n<pre class=\"codehilite\"><code class=\"language-json\">[\n  {\n    &quot;Config&quot;: &quot;8f3a2b1c...json&quot;,\n    &quot;RepoTags&quot;: [&quot;our-app:latest&quot;],\n    &quot;Layers&quot;: [\n      &quot;sha256:a3ed95caeb02...&quot;,\n      &quot;sha256:5f70bf18a086...&quot;,\n      &quot;sha256:7c3d1f2b4e5a...&quot;\n    ]\n  }\n]\n<\/code><\/pre>\n<p>Each of those SHA-256 hashes represents a layer. The hash is generated by taking the content of the layer&#8217;s tarball and running it through the SHA-256 algorithm. This is &#8220;content-addressable storage.&#8221; If the content changes by even one bit\u2014say, you added a single space to a YAML file\u2014the hash changes completely.<\/p>\n<p>The junior dev\u2019s mistake was thinking that by &#8220;overwriting&#8221; the <code>latest<\/code> tag, they were replacing the image. No. They were just adding more garbage to the stack. The registry now has to store every single version of those 4GB layers because the &#8220;latest&#8221; tag is just a mutable pointer. It\u2019s a lie. It\u2019s a shortcut that leads directly to production outages.<\/p>\n<hr \/>\n<h3><span class=\"ez-toc-section\" id=\"3_Deconstructing_the_Manifest_Content-Addressable_Chaos\"><\/span>3. Deconstructing the Manifest: Content-Addressable Chaos<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>When we talk about <strong>what is<\/strong> an image identity, we are talking about the DiffID and the ChainID. This is where people get lost in the YAML indentation hell of their own making. <\/p>\n<p>The Docker Engine v25.0.3 uses these hashes to ensure integrity. When a layer is downloaded, Docker calculates the SHA-256 of the compressed artifact (the Distribution Hash). Then, it decompresses it and calculates the hash of the uncompressed content (the DiffID). <\/p>\n<p>If you run <code>docker inspect<\/code> on the bloated image, you see the horror:<\/p>\n<pre class=\"codehilite\"><code class=\"language-bash\">$ docker inspect our-app:latest\n[\n    {\n        &quot;Id&quot;: &quot;sha256:8f3a2b1c...&quot;,\n        &quot;RepoTags&quot;: [\n            &quot;our-app:latest&quot;\n        ],\n        &quot;RepoDigests&quot;: [\n            &quot;our-app@sha256:d4e5f6...&quot;\n        ],\n        &quot;Parent&quot;: &quot;&quot;,\n        &quot;Comment&quot;: &quot;buildkit.dockerfile.v0&quot;,\n        &quot;Created&quot;: &quot;2023-10-14T03:00:00Z&quot;,\n        &quot;Container&quot;: &quot;...&quot;,\n        &quot;DockerVersion&quot;: &quot;25.0.3&quot;,\n        &quot;Author&quot;: &quot;&quot;,\n        &quot;Config&quot;: {\n            &quot;Env&quot;: [\n                &quot;PATH=\/usr\/local\/sbin:\/usr\/local\/bin:\/usr\/sbin:\/usr\/bin:\/sbin:\/bin&quot;\n            ],\n            &quot;Cmd&quot;: [\n                &quot;python&quot;,\n                &quot;app.py&quot;\n            ],\n            &quot;Image&quot;: &quot;sha256:8f3a2b1c...&quot;\n        },\n        &quot;Architecture&quot;: &quot;amd64&quot;,\n        &quot;Os&quot;: &quot;linux&quot;,\n        &quot;Size&quot;: 4294967296,\n        &quot;GraphDriver&quot;: {\n            &quot;Data&quot;: {\n                &quot;LowerDir&quot;: &quot;\/var\/lib\/docker\/overlay2\/layer_a\/diff:\/var\/lib\/docker\/overlay2\/layer_b\/diff&quot;,\n                &quot;MergedDir&quot;: &quot;\/var\/lib\/docker\/overlay2\/layer_c\/merged&quot;,\n                &quot;UpperDir&quot;: &quot;\/var\/lib\/docker\/overlay2\/layer_c\/diff&quot;,\n                &quot;WorkDir&quot;: &quot;\/var\/lib\/docker\/overlay2\/layer_c\/work&quot;\n            },\n            &quot;Name&quot;: &quot;overlay2&quot;\n        },\n        &quot;RootFS&quot;: {\n            &quot;Type&quot;: &quot;layers&quot;,\n            &quot;Layers&quot;: [\n                &quot;sha256:a3ed95caeb02...&quot;,\n                &quot;sha256:5f70bf18a086...&quot;,\n                &quot;sha256:7c3d1f2b4e5a...&quot;\n            ]\n        },\n        &quot;Metadata&quot;: {\n            &quot;LastTagTime&quot;: &quot;2023-10-14T03:05:00Z&quot;\n        }\n    }\n]\n<\/code><\/pre>\n<p>Look at that <code>Size<\/code> field. 4.2 billion bytes. Most of that is dead weight. Because the developer used Debian Bookworm as a base instead of something sane like Alpine 3.19, they started with a 100MB+ footprint before they even wrote a line of code. Then they piled on the layers. <\/p>\n<p>The <code>RootFS.Layers<\/code> array shows the stack. Each hash here is a DiffID. If you want to know why your builds are slow, it\u2019s because Docker has to calculate these hashes every single time you change a line of code and rebuild. It\u2019s a massive CPU tax paid for the privilege of laziness.<\/p>\n<hr \/>\n<h3><span class=\"ez-toc-section\" id=\"4_The_Overlay2_Autopsy_Where_the_Files_Actually_Live\"><\/span>4. The Overlay2 Autopsy: Where the Files Actually Live<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>To understand <strong>what is<\/strong> happening on the filesystem, we have to look at <code>\/var\/lib\/docker\/overlay2\/<\/code>. This is the &#8220;Graph Driver.&#8221; It\u2019s where the magic\u2014and the nightmares\u2014happen. <\/p>\n<p>When you pull an image, Docker creates a directory for each layer. It uses the <code>overlay2<\/code> storage driver to mount these layers on top of each other. This isn&#8217;t a simple copy. It uses the <code>mount<\/code> syscall with the <code>overlay<\/code> type. <\/p>\n<p>The kernel takes the <code>LowerDir<\/code> (the read-only layers), the <code>UpperDir<\/code> (the changes made in the current layer), and presents them as a single, unified filesystem in the <code>MergedDir<\/code>. <\/p>\n<p>During the incident, I had to manually inspect the <code>diff<\/code> directories to find out where the 4GB was coming from. <\/p>\n<pre class=\"codehilite\"><code class=\"language-bash\"># Navigating the graveyard of disk space\n$ cd \/var\/lib\/docker\/overlay2\/\n$ du -sh * | sort -hr | head -n 5\n4.2G    l\/6W7Q...\n1.2G    l\/2B9R...\n...\n<\/code><\/pre>\n<p>The <code>l<\/code> directory contains symbolic links to the actual layer IDs to avoid hitting the kernel&#8217;s limit on mount command lengths. If you look inside one of these <code>diff<\/code> folders, you see the actual files. In our case, I found a <code>\/root\/.cache\/pip<\/code> directory that was 1.5GB. Why? Because the dev didn&#8217;t use <code>--no-cache-dir<\/code> in their Dockerfile. <\/p>\n<p>This is the reality of Docker: it\u2019s just a clever way of lying to the process about where its files are. But the disk doesn&#8217;t lie. The disk just fills up until the kernel starts killing processes to save itself.<\/p>\n<hr \/>\n<h3><span class=\"ez-toc-section\" id=\"5_The_%E2%80%9CLatest%E2%80%9D_Sin_and_the_Immutable_Myth\"><\/span>5. The &#8220;Latest&#8221; Sin and the Immutable Myth<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>We need to talk about the &#8220;latest&#8221; tag. In a sane world, we would use immutable digests. A digest looks like this: <code>our-app@sha256:d4e5f6...<\/code>. This is a permanent, unchangeable reference to a specific manifest.<\/p>\n<p>The &#8220;latest&#8221; tag is a pointer. It\u2019s like a DNS record that someone can change at any moment. When the junior dev pushed their 4GB image with the <code>latest<\/code> tag, every node in the cluster that had a <code>PullPolicy: Always<\/code> (another mistake) immediately tried to pull it. <\/p>\n<p>Because the tag was the same, but the underlying SHA-256 digest had changed, Docker Engine v25.0.3 realized its local cache was invalid. It began the &#8220;Death Pull.&#8221; <\/p>\n<p>The OCI (Open Container Initiative) Image Specification defines how these tags and digests work. A tag is just a reference in the registry&#8217;s key-value store. It points to a manifest. The manifest points to the layers. By using <code>latest<\/code>, you are essentially saying, &#8220;I want whatever random garbage was most recently uploaded to this name.&#8221; <\/p>\n<p>In production, this is suicide. We use specific version tags or, better yet, the SHA-256 digest of the image. If we had been using digests, the new 4GB image would have sat harmlessly in the registry until we explicitly updated the deployment manifest. Instead, it was a forced injection of bloat.<\/p>\n<hr \/>\n<h3><span class=\"ez-toc-section\" id=\"6_The_4GB_Bloat_A_Forensic_History\"><\/span>6. The 4GB Bloat: A Forensic History<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>I ran <code>docker history --no-trunc<\/code> on the offending image. This command is the only way to see the &#8220;why&#8221; behind the &#8220;what.&#8221; It shows the commands that created each layer.<\/p>\n<p>&#8220;`text<br \/>\nIMAGE          CREATED          CREATED BY<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Related_Articles\"><\/span>Related Articles<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Explore more insights and best practices:<\/p>\n<ul>\n<li><a href=\"https:\/\/itsupportwale.com\/blog\/latest-lts-version-of-ubuntu-in-2020\/\">Latest Lts Version Of Ubuntu In 2020<\/a><\/li>\n<li><a href=\"https:\/\/itsupportwale.com\/blog\/how-to-block-microsoft-bookings-access-in-tenant\/\">How To Block Microsoft Bookings Access In Tenant<\/a><\/li>\n<li><a href=\"https:\/\/itsupportwale.com\/blog\/fixed-nginx-showing-blank-php-pages-with-fastcgi-or-php-fpm\/\">Fixed Nginx Showing Blank Php Pages With Fastcgi Or Php Fpm<\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>INCIDENT REPORT #9902-B: THE DEATH OF OUR REGISTRY Date: October 14, 2023 Duration: 48 Hours (and my sanity) Status: Post-Mortem \/ Educational Intervention Lead SRE: [REDACTED] 1. Summary of the Carnage At 03:14 UTC, the monitoring stack for our production cluster started screaming. Node prod-worker-04, running Docker Engine v25.0.3, reported a total disk exhaustion event &#8230; <a title=\"What is a Docker Image? A Complete Guide for Beginners\" class=\"read-more\" href=\"https:\/\/itsupportwale.com\/blog\/what-is-a-docker-image-a-complete-guide-for-beginners\/\" aria-label=\"Read more  on What is a Docker Image? A Complete Guide for Beginners\">Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-4720","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.0 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>What is a Docker Image? 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