ci: Build the CI trigger container as part of b2c

See mesa/mesa!14660 (comment 1228758) for the original suggestion. A userland will be needed from various projects in Gitlab CI to submit b2c jobs to the executor. It's a little questionable whether this container should live in the boot2container namespace or the valve-infra namespace, since the requirements this container honours are split between the projects. At least now it's automatically built :)