Revamp CI

[spilchen]

This is a major overhaul of the CI:

• We now build all of the images first and store them in the GitHub container registry. The e2e tests in turn use the container registry, previously they each would create the containers themselves. The main benefit is that we preserve the containers that we tested with so that we can release the actual containers that were used in our CI.
• Add security vulnerability scanning of the images. Scanning doesn't fail the built. But findings from the scans are now an artifact provided by the build and and can be found in the Security tab of the repo.
• Allow the CI to be run with pre-built containers that are stored in a private repostiory in dockerhub. This will allow us to run the CI against unreleased versions of the Vertica server.
• Add some standard labels to the containers