Add new runner for running docker-in-docker to build containers

Creating a new runner just for building container. This one will be privileged and allow to run docker with TLS. It is to run only on "protected" branches so that we don't need to expose "privileged" docker runner to every build.

Running with TLS available mean that we can run "docker" command in a jobs without setting any variables for a job.

The new runner should only used for building containers, so it should be fine to keep doing builds on the original runner at the same time in term of resources. So, set "runners_concurrent" to 2.

Back to unregister "--all-runners" as this should only unregister runner from one machine.