Add SSH support for rootless docker

[aidanmoss]

Currently attempting to use the "--ssh-dir" option will import a directory into a docker container that is still owned by the root user of the container.

This does not work as "builder" cannot access these files making this option useless in a rootless environment. This PR adds a small change to check for the existence of the ssh directory if it exist and changes its ownership to the appropriate user.

[jan-kiszka]

Thanks for the report and the fix proposal. Could you send it to our mailing list (see https://github.com/siemens/kas/blob/master/CONTRIBUTING.md)?

I'm now wondering if this is really the only remaining case. Did you check that as well?

[aidanmoss]

Something odd is happening with my 'git send-email` this morning, but just sent the patch to the mailing list.

I know the ssh-agent runs into a similar issue as well but I don't have a solution for it at the moment, I'll look into the other options as well and see if other ones also have this problem.