Add Docker container that is automatically build for releases

[sharkwouter]

I need a gscloud docker container for something I'm working on right now and I thought this might be useful to do upstream, so others also benefit from this work.

This adds the following changes:

• Whenever a tag is created, build a docker container
• Tag this container as both latest and the tag name
• Push both tags to the Docker registry on this GitHub repo

The container has gscloud as entrypoint. Here is a quick example on how you'd use it:

$ docker run -ti --rm ghcr.io/sharkwouter/gscloud:latest version
Version:        test13
Git commit:     1abd35253f6ed698f4db4c3d1ac206279e0156c9

[nvthongswansea]

LGTM