[bug] limit docker job to only run in main repository

This will enable forks to run jobs, without needing access to the private token, or access to the docker-in-docker tagged runner.

As it is right now, @simonft cannot make the pipeline pass because the docker image creation job is trying to run, and it cannot run because the required tagged runner is not available to him. This job should only run when it is actually run in the canonical project, and not run for individual forks. This commit adds the 'only' regexp to limit the job to only run in that situation, which will cause the job to not be even queued for forks.