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Debug

how to debug docker build

  • --no-cache: forece docker to rebuild all layers from scratch

  • --progress=plain: disable progress indicator and get more detailed output

  • --verbose: enable verbose output to identify the root cause of the failure

  • check system logs for system-level issues

  • run the container interactively

https://stackoverflow.com/questions/26220957/how-can-i-inspect-the-file-system-of-a-failed-docker-build

get image layer id (hash)

Build Kit can make your build faster but doesn't support intermediate container hashes: https://github.com/moby/buildkit/issues/1053

DOCKER_BUILDKIT=0 docker build ...
Changed to DOCKER_BUILDKIT=plain???

windows terminal set Var=val, powershell $env:Var=val.

Example (the id is the one like ---> d62bcfd4677c not the Running in)

Step 1/5 : FROM busybox
 ---> d62bcfd4677c
Step 2/5 : SHELL ["/bin/bash", "-c"]
 ---> Running in 8058f86dcb62
Removing intermediate container 8058f86dcb62
 ---> c5a1a5c423e6
...

run into last succsessful layer

Everytime docker successfully executes a RUN command from a Dockerfile, a new layer in the image filesystem is committed. We can use those layers ids as images to start a new container.

Start a new container from layer id, then can try any commands, including the commanf failed.

docker run --rm d62bcfd4677c cat /tmp/foo.txt   #run a command
docker run --rm -it d62bcfd4677c0 sh            #start a shell
docker run --rm -it d62bcfd4677c0 bash -il

  • --rm: This option tells Docker to automatically remove the container when it exits. This is useful for cleaning up containers that are no longer needed, especially when running one-off tasks.

  • -il: This instructs Docker to start an interactive Bash shell inside the container and to treat it as if it were a login shell, which means it will execute the login shell startup files to set up the environment.

run into the failed layer

find the container that failed:

docker ps -a
Commit it to an image: 
docker commit <container-id>
And then run the image (if necessary, running bash): 
docker run -it <new-image-id> [bash -il]

Why need to create a new image from the container?

  • start the failed container again it would run the command that failed again, and you'd be back where you started.

  • By creating an image you can start a container with a different start command.