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Lazy coder

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How to debug image build with Dockerfile

How to debug image build with Dockerfile

Dockerfile to build the env for Ruby on Rails App 

FROM ruby # install essential libs/commands RUN apt-get update && apt-get install lib-ssl ... # install gems RUN bundle install ... # install npm RUN yarn install COMMAND bundle exec rails server
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  1. delete the following part causing errors 
FROM busybox RUN echo 'hello world' > /tmp/test RUN exit 1 .... RUN echo 'ready'
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RUN exit 1
Just remove RUN exit 1 and the following

  1. run intermediate image with sha Turn off buildkit to get the sha for each layer docker run --rm -it current sh to debug 
❯ DOCKER_BUILDKIT=0 docker build -t test . Sending build context to Docker daemon 2.048kB Step 1/3 : FROM busybox ---> abaa813f94fd Step 2/3 : RUN echo 'hello world' ---> Using cache ---> 551ba8324834 Step 3/3 : RUN exit 1 ---> Running in 5e3c523c38a3 The command '/bin/sh -c exit 1' returned a non-zero code: 1
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# pick sha of last successful layer docker run --rm -it 551ba8324834 sh
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  1. nsenter to debug

Enter into the name space of the process.
Namespaces are a feature of the Linux kernel that partitions kernel resources such that one set of processes sees one set of resources while another set of processes sees a different set of resources.

  • add RUN sleep infinite to Dockerfile
  • docker run -it --rm --privileged --pid=host justincormack/nsenter1 to get to the building host 1
  • ps -ef|grep sleep to find pid
  • nsenter -p -m -u -i -n -t 10012 sh 
FROM busybox RUN echo 'hello world' RUN sleep infinite RUN exit 1
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❯ docker build -t test . [+] Building 10.7s (5/7) => [internal] load build definition from Dockerfile 0.0s => => transferring dockerfile: 73B 0.0s => [internal] load .dockerignore 0.0s => => transferring context: 2B 0.0s => [internal] load metadata for docker.io/library/busybox:latest 0.0s => [1/4] FROM docker.io/library/busybox 0.0s => CACHED [2/4] RUN echo 'hello world' 0.0s => [3/4] RUN sleep infinite
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  1. use target to build sucessfully the image multiple-stage builds 
FROM busybox as working RUN echo 'hello world' FROM working as error RUN exit 1
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# build image with target docker build -t test --target working . # run image to debug docker run --rm -it test sh
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docker run --privileged --pid=host -it alpine \ nsenter -t 1 -m -u -n -i sh
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  • --rm : removes the container after it is stopped -ti (or -t -i) : adds a tty and leaves the standard input opened
  • --privileged : grants additional permissions to the container, it allows the container to gain access to the devices of the host (/dev)
  • --pid=host : allows the containers to use the processus tree of the Docker host (the VM in which the Docker daemon is running)

check pid 

❯ docker run -ti --rm busybox sh ❯ docker ps ❯ docker inspect --format '{{.State.Pid}}' a57c56a83e54 18762
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  1. althernative way using alpine and nsenter command ↩

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