DOCKER-357: Move build from Travis to GitHub actions

[koenvangeerteruy]

Hello @tgeens,

The build has been moved from Travis to GitHub Actions.

There are 2 workflows:

• Continuous Integration: running on push and pull requests for all branches
• Scheduled Build: running on a schedule and building master and release branch

The scheduled build is only triggered from the default branch (master). This is currently a limitation in GitHub Actions, see GitHub documentation: workflow syntax for on.schedule and Scheduled Events. So for building multiple branches I used strategy and matrix on the branches we want to build to trigger multiple builds.

The schedule is set to run every Saturday at 00:07 UTC.

For the scheduled build I introduced additional timestamped tags, as is also the case for Ubuntu and Debian. This is to be able to trace back when an updated distribution package could have broken something.

I prepared this PR to support/use the master-release release approach.
Publishing of docker images is enabled on master and release branches. So to not overwrite tagged release images with master (development) images, I introduced the -SNAPSHOT tag qualifier for master branch images.

Credentials for the image publishing account should be stored in GitHub repository secrets:

• DOCKER_USER: the account to publish images to the Xenit organisation on Docker Hub
• DOCKER_PASSWORD: the access token created for docker-openjdk on Docker Hub

To have release tags and not break the scheduled build a release branch needs to be created. The master branch image tags will be postfixed with -SNAPSHOT
This release branch could be created from the current master branch.

Best regards,
Koen