Use a GitHub Actions workflow to deploy buildenv

[djungelorm]

Use a GitHub Actions workflow to deploy new versions of the buildenv docker image.

This workflow should be triggered when a change is pushed to the tools/docker/VERSION.txt file.

Doing it this way means that we don't need to locally build, then push a very large image. We just push the code changes, and let GitHub Actions do the build.

[GaryDeco]

I see. That seems much more efficient.

[djungelorm]

Also easier for someone to update the image without needing to have docker installed locally.