Introduce DockerHub build trigger

[hanefi]

We have several rules on Docker Hub that gets triggered on branch and tag pushes. However we do not have a mechanism to update images nightly. This PR aims to do the following:

• Introduce nightly image creation in test steps
• Provide clear and understandable names that help improve CI interface
• Create a mechanism to update the docker images every day

Proposed CI workflow:

• Run 3 jobs in parallel, create latest, alpine and nightly images
• Trigger Docker Hub builds only when:
  • on master branch
  • all the images were successfully built

[hanefi]

Why I removed the env fields here:

1. The trigger-dockerhub stage should happen only once, not 3 times
2. It is not so easy to provide name fields, when we have a matrix (If name field is provided they significantly improve CI interface)

[hanefi]

CI docs state that branch variable stores current branch name; However for pull requests: the base branch name.

We should trigger builds only in two cases:

1. A PR gets merged into master
2. A Cron Job triggers the CI build on master

[JelteF]

LGTM, assuming you did some testing of this.

[hanefi]

LGTM, assuming you did some testing of this.

Yes, I did. I monitored recent builds on docker hub, and observed that we can build new images there.