Trying to run "comitted" Docker image, get "cannot mount volume over existing file, file exists"

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I am developing a Docker image. I started with a base image and was working inside it interactively, using bash. I installed a bunch of stuff, and the install (which included compiling a lot of code) took over 20 minutes, so to save my work, I used:

$ docker commit 0f08ac958391 myproject:wip

Now when I try to run the image:

$ docker run --rm -it myproject:wip
docker: Error response from daemon: cannot mount volume over existing file, file exists /var/lib/docker/overlay2/95aa9a9ea7cc0b1ba302adbd287e4d7059ee4fbe64183404df3bc65df332ee63/merged/run/host-services/ssh-auth.sock.

What is going on? How do I fix this?

Note about related/duplicate questions: while there are other questions about this error message, none of the answers directly explain why the error happens in this situation or what to do about it. In fact, most of the questions have no answers at all.

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1 Answers

When I ran the base image, I included a mount for the SSH agent socket:

$ docker run --rm -it -v /run/host-services/ssh-auth.sock:/run/host-services/ssh-auth.sock myproject:dev /bin/bash

This bind mounts a file from the host (actually the Docker daemon VM) to a file in the Docker container. When I committed the running image, the image contained the file /run/host-services/ssh-auth.sock. The image also contained an empty volume reference to /run/host-services/ssh-auth.sock. This means that when I ran

$ docker run --rm -it myproject:wip

It was equivalent to running

$ docker run -v /run/host-services/ssh-auth.sock --rm -it myproject:wip

Unfortunately, what that command does is create an anonymous volume and mount it into the directory /run/host-services/ssh-auth.sock in the container. This works if the container has such a directory or even if it does not. What causes it to fail is if the target name is taken up by a file. Docker will not mount a volume over a file.

The solution is to explicitly provide a mapping from a host file to the target volume. Any host file will do, but in my case it is best to use the original. So this works:

docker run --rm -it -v /run/host-services/ssh-auth.sock:/run/host-services/ssh-auth.sock myproject:wip
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