Introduce nightly release

What does this MR do?

• The release job can now be utilised to create a release or perform a nightly release which is just rebuilding the images and pushing them.
• The release script will
  • first tag and push images (Nightly release or release mode)
  • if NIGHTLY_RLEASE env var is not set it will also create a git tag and a gitlab release. (release mode)
• nightly release runs only in scheduled pipeline. If CI_PIPELINE_SOURCE is schedule then we set NIGHTLY_RELEASE to true

Why are we doing this?

We want to update the base image daily. The trivy image currently used extends the alpine image. We rely on trivy itself to fix vulnerabilities of the base image. In order to support quick vulnerability fixes in the base image we use alpine as a base instead of the trivy image. This way we can rebuild the image daily and get the latest version of the base.

What are the relevant issue numbers?

gitlab-org/gitlab#444470 (closed)