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Dimitrios Desyllas
Dimitrios Desyllas

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Get a debug console on failing docker container launches.

#howto

Using docker compose I am building an image:

docker compose build someservice
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Or directly via docker for example:

docker build .
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But when I try to launch it:

docker compose up -d
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But for some reason is unable to be launched and it is unexpectedly terminated.

Via docker ps -a we detect our failing docker container for example:

$ docker ps -a

CONTAINER ID   IMAGE                           COMMAND                  CREATED              STATUS                      PORTS                                                             NAMES
67a0f27524a6   blog-testprod-django-testprod   "/bin/sh /app/entryp…"   About a minute ago   Exited (127) 1 second ago                                                                     blog-testprod-django-testprod-1
54d03ad72f70   nginx:stable-alpine             "/docker-entrypoint.…"   39 minutes ago       Up 38 minutes               0.0.0.0:8001->80/tcp, [::]:8001->80/tcp                           blog-testprod-nginx-1
99864e6ab6a5   axllent/mailpit                 "/mailpit"               39 minutes ago       Up 39 minutes               1025/tcp, 1110/tcp, 0.0.0.0:8026->8025/tcp, [::]:8026->8025/tcp   blog-testprod-mailer-1
6232c1fdeaea   postgres:17                     "docker-entrypoint.s…"   39 minutes ago       Up 39 minutes (healthy)     0.0.0.0:5432->5432/tcp, [::]:5432->5432/tcp                       blog-testprod-postgresql-1
d0d614339873   moby/buildkit:buildx-stable-1   "buildkitd --allow-i…"   13 months ago        Up 13 hours                                                                                   buildx_buildkit_loving_hawking0
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As we can see the container blog-testprod-django-testprod-1 is failing and we need to diagnose why it happens. For that we need to access a console in order to run our diagnostic commands.

For that we commit it into a new image:

docker commit blog-testprod-django-testprod-1 debug-image
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And then we run overriding the entrypoint with a shell. I our case we are using /bin/bash:

docker run -it --rm --entrypoint /bin/bash debug-image
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The entrypoint override happend via --entrypoint param.

If /bin/bash is not available use /bin/sh instead:

docker run -it --rm --entrypoint /bin/sh debug-image
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We do not need to provide any further arguments after debug-image whebn executing command above. Using shell as an entrypoint alongside the -ti we receive an interactive shell.

Now we are able to execute any command nessesary in order to diagnose any problem, the commands needed for problem diagnosis depend on each case.

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