Add docker debugging post

content/posts/2022-09-30-debugging-docker-containers.md

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+---
+title: Debugging Docker containers
+date: "2022-09-30"
+tags: "note, 100DaysToOffload, guide, programming, docker"
+---
+
+Docker builds images incrementally. Every line in a Dockerfile will generate a
+new image that builds on top of the last one. This can be really handy if
+something is not right in your build.
+
+Since version 18.09 Docker has added a new backend for building images,
+[buildkit](https://github.com/moby/buildkit#buildkit). Unfortunately, buildkit
+does not let you view the IDs of the intermediate containers it uses under the
+hood. To work around that, you can opt out of buildkit by running a build with
+buildkit disabled:
+
+```sh
+DOCKER_BUILDKIT=0 docker build --pull --rm -t myproject:latest .
+```
+
+You should now see the IDs of the intermediate containers:
+
+```sh
+Sending build context to Docker daemon  87.84MB
+Step 1/16 : FROM node:16.15.1-alpine3.16 AS development
+16.15.1-alpine3.16: Pulling from library/node
+Digest: sha256:c785e617c8d7015190c0d41af52cc69be8a16e3d9eb7cb21f0bb58bcfca14d6b
+Status: Image is up to date for node:16.15.1-alpine3.16
+ ---> e548f8c9983f
+Step 2/16 : WORKDIR /usr/src/app
+ ---> Using cache
+ ---> 34e5c9bdb910
+Step 3/16 : COPY package*.json ./
+ ---> Using cache
+ ---> 626e4ae998fc
+Step 4/16 : RUN npm install glob rimraf
+ ---> Using cache
+ ---> 2d036b8354e0
+Step 5/16 : RUN npm install
+ ---> Using cache
+ ---> 948709b4957f      <-- HERE
+Step 6/16 : COPY . .
+...
+```
+
+As mentioned, these IDs are valid docker images, so you can just launch them
+and attach a shell like every other image:
+
+```sh
+docker run -ti --rm 948709b4957f
+```
+
+If you're not seeing a regular shell, but a Node.js REPL for example, this
+might be because the `ENTRYPOINT` of that image was set to the binary of that
+REPL. To work around that, you can override the entrypoint:
+
+```sh
+docker run -ti --rm --entrypoint=/bin/sh 948709b4957f
+```
+
+## When is this helpful?
+
+If your build fails at a particular step, you can attach a shell to the **last
+working** step, inspect the filesystem, and execute the failing command manually.
+
+That's all!
+
+This is post 039 of [#100DaysToOffload](https://100daystooffload.com/).