Build multi arch images on GitHub Actions and push to DockerHub

[buchdag]

As proposed in #346 (comment), this PR adds a GitHub Actions workflow that build multi arch images of jwilder/docker-gen and push them to DockerHub.

For this PR to work, in addition to enabling GitHub Actions, two secrets need to be added to the repo:
DOCKERHUB_USERNAME: the name of DockerHub account that has push rights to jwilder/docker-gen
DOCKERHUB_TOKEN: a DockerHub access token for the DOCKERHUB_USERNAME account

The workflow is triggered:

• on push to the master branch
• on creation of a new x.y.z semver tag
• on schedule every Monday at 00:00 UTC

It can also be triggered manually thanks to workflow_dispatch

The images created from new Git tags are tagged as :{{major}}.{{minor}} and :{{major}}.{{minor}}.{{patch}}, ie a new 0.7.6 tag would push jwilder/docker-gen:0.7.6 and jwilder/docker-gen:0.7 to Dockerhub.

Every newly built image is also pushed as latest.

[jwilder]

I setup the secrets.