Add github workflow to build containers

[yselkowitz]

This workflow should automatically push a multi-arch container image build whenever a tag is created.

Fixes: #324

[rnsc]

@yselkowitz would it make sense to build without push on PR to check that it builds properly before merging?

[yselkowitz]

That would be up to the maintainer, but 1) it looks like Travis CI is already being used for checking the code compilation, 2) it might be an overkill to cross-build the container for every PR (maybe that should be limited to Dockerfile changes only?), 3) the maintainer's Github Actions quota needs to be considered, and 4) perhaps to avoid abuse some protection should be put in place to avoid triggering CI builds for first-time contributors.

[yselkowitz]

/cc @edenhill