Heptapod CI: make sure Docker jobs runner can push to registry

We have currently to avoid using the Clever Runner for the packaging
job, meaning that we reinstate the shared runners for the Omnibus
Heptapod project. In turn, this means that we must protect the
docker-testing job with the `container-registry-push` tag.

When the situation gets back to normal, we shouldn't need to change
anything, because the Clever group-level Runner also has the tag.

--HG--
branch : heptapod-stable
4 jobs for !47 with topic/heptapod-stable/bump-stable in 104 minutes and 36 seconds (queued for 4 seconds)
latest detached
Status Name Job ID Coverage
  Test
passed rspec #253312

00:03:15

 
  Package
passed pkg-ubuntu-focal #253313

01:37:39

 
  Docker
passed docker-testing #253314
container-registry-push

00:03:41

 
  Functional
passed heptapod-tests-stable