Dynamically pick Docker image based on branch name

Look at the branch name to pick the Docker image from the matching release. For instance, if the pipeline is run on the apertis/v2020 branch, choose the v2020-package-source-builder image.

Also do the same if the branch name is apertis/v2020-security, and if the branch name is wip/em/land-to-v2021dev3-this-patch use the v2021dev3-package-source-builder image.

If no release name is specified in the current branch name, fall back to extracting it from the name of the default branch.