docker-credential-pass has wrong $HOME directory when running docker with sudo

I'm not overly familiar with this repo, but I've just wrestled with docker-credential-pass for an hour and now I've figured out the problem it seems quite fixable.

The error is this: Error saving credentials: error storing credentials - err: exit status 1, out: pass store is uninitialized.

Here's how this problem can occur:

1. The user initialises their pass store with pass init, creating the ~/.password-store directory.
2. They download docker-credential-pass and add it to their $PATH.
3. They attempt to login with sudo docker login (not using sudo would cause permission errors).
4. The login succeeds but with the above error - their credentials haven't been saved, and it claims pass store is uninitialized even though it definitely is.

The cause is that, in some configurations of Linux, the root user has a different ~ (home directory) to the normal user. This means docker-credential-pass is looking in the wrong place for the pass store; on my machine I believe it was looking in /root/.password-store. I fixed this on my own machine by editing /etc/sudoers so that the root user keeps the $HOME env variable of the normal user, but it seems that the program could be edited in some way to avoid this requirement altogether, or at least to provide a more useful error message. If I was familiar with Go I would attempt to make this change myself.

Note: this may be the cause of issue #140 , I'm not sure.

The issue names no files or tests. Start by tracing docker-credential-pass handling of $HOME and the pass store, then compare the possible relationship with issue #140. Done means the behavior under sudo is defined and either avoids the wrong pass-store location or provides a more useful error.