Adds a new build variable to force an update of the Docker container.
This variable can be set in a scheduled job to update the Docker image
periodically.
This way the Prefactor pipeline will keep using the latests versions of
its underlying software packages.

This command requires experimental cli to be enabled or using the
lastest Docker version. Other repositories have opted for the latter
solution so this repository follows suit.

For each of the packages that are cloned from Git it is now possible to specify a branch or tag name. Also, the list of packages installed in the "runner" build was cleaned up to reduce the size of the final container image.