Publish Docker image

[abitrolly]

Fixes #82.

This publishes to GitHub container registry, because it doesn't need any additional configuration.

This is what was built in my fork https://github.com/abitrolly/copybara/pkgs/container/copybara

[abitrolly]

@cpovirk do you know anybody who can merge this?

[cpovirk]

I've submitted a few changes for documentation or (more often) larger migrations that happen to touch Copybara, but I didn't have context on the project in general :(

[abitrolly]

@cpovirk is there any DRI (directly responsible person) for this project? Maybe GitHub folks or Google OSS folks can help?

[hsudhof]

Thank you for sending in the contribution, we appreciate the input!
Unfortunately, we are not currently equipped to offer formal releases and the support this entails at the moment, we hope this will change in the future. We also do not use actions at present and would use a GCB based approach.

[abitrolly]

GCB is https://cloud.google.com/build