I work with docker containers pretty much every day and debugging containers that crash immediately on startup has been a common headache for me. I came up with a little “hack” that has helped me with this problem so I thought I’d share it here.
The problem
Debugging containers is relatively easy when they’re up or online! You can open a shell in the container and peek around to see what’s happening, just like you would on your machine. The following command will open up a simple shell in most containers:
docker compose exec -it /bin/sh my-job
The problem is, if your container won’t come up at all (or it immediately crashes when it does come up) you can’t open a shell in it.
This makes sense. The container isn’t online, so you can’t run anything inside of it! Try it, and you’ll get this message:
service "my-job" is not up
My solution
My workaround is to replace the startup command for the container with the sleep command.
services:
my-job:
container: my-container:local
command: sleep infinity; # this line will replace the startup command
Instead of starting as per the entrypoint or command that is set in the container, the container’s main “thread” will go into sleep for an infinite amount of time. Usually you’d never want to do this. However, this will effectively keep your container online and “healthy” until you take it down yourself, which is exactly what we want!
Then you can just startup your container, open a shell, and start debugging:
docker compose up my-job
docker compose exec -it /bin/sh my-job