Add Cerbero CI docker and scripts

Now that the required changes to Cerbero has been merged, we can now start progressively enabling Cerbero CI. In this merge request you will find the scripts needed to create the new docker image. The docker is generated from a cerbero bootstrap, so that we can properly verify that our bootstrap is complete. It also contains a cerbero deps job, this will be used in cerbero/ project to build all the dependency and speed up the cerbero jobs. Not having the dependency does not break the build on purpose. Finally, the local build is enabled, so that the CI makes a build using the generated Docker image.

The other projects builds is commented out, since we need to know the docker SHA. Also, we will only enable it when we have the cerbero/ CI ready with the deps cache generated (so that we don't slow down all builds).

For reviewers: I had to rebase this work a lot, as it seems we changed our mind a lot on how we wanted our scripts. I might have missed some of the "new ways". I didn't split out scripts form the docker files, as what I have is working fine and didn't see any benefit for now. I guess it can be done later while enabling the other builds. Also, the current CI is considerably slower then what we'll get later on when the cerbero/ cache is available.