WIP: Test with docker image

WIP: testing CI with docker image

• Remove anything not associated to the changes in the CI (it seems like only .gitlab-ci.yml should stay)
• Probably the correct execution relies on cvmfs being mounted in the gitlab runner. Check whether we need to use runners with cvmfs tag https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/settings/ci_cd (we do)
• Check if we need to use atlas_external_cvmfs image or if it's enough to use tags:cvmfs
• Check whether we need to do anything specific for GRID files that live on eos. Do we need the eos image to run the job?
• Build docker image from atlas_external_cvmfs and gitlab-registry.cern.ch/atlas-physics/pmg/mcjoboptions:eos_bash
• Check whether slc6 or cc7 is used (most probably it's the former but good to check - then #33 (closed) can be closed)
• Use this new image to test with
  • 421001 (simple jO)
  • 421003 (contains GRID file)

Use only 10 events for testing