feat: add image user option

[maxbrunet]

Hello! :wave:

When using a base image that does not set a user (e.g. busybox), it is desirable to set a non-root user for the built image. At the moment, this requires building a custom image built with another tool to set this value. Since it is a simple config field, it seems reasonable to have an option for it

[rsrchboy]

This would be incredibly useful for a project I'm working on.

[github-actions[bot]]

This Pull Request is stale because it has been open for 90 days with no activity. It will automatically close after 30 more days of inactivity. Keep fresh with the 'lifecycle/frozen' label.

[maxbrunet]

Any chance to get some feedback on this PR? :pray:

[rsrchboy]

I can't do an official review, but going off of CONTRIBUTING (and common sense) things LGTM. Tests are included and pass; --image-user behaves as expected.

CI appears to be complaining about license issues, but apparently the "details" link isn't public, so I don't know what's going on there. Given that this PR does not introduce any library changes, I'm inclined to think that these are preexisting issues.

[imjasonh]

This change looks good to me! Can you rebase so that CI runs and I can approve and merge? 👍

[maxbrunet]

This change looks good to me! Can you rebase so that CI runs and I can approve and merge? 👍

Done! :heart:

[HeavyWombat]

Looking forward to testing this out.