CI: remove outdated 'docker' tab to run on the new CERN infrastructure

The CERN GitLab CI infrastructure has changed in the latest months, and runners tagged as 'docker' are being phasing out.

To run our jobs on the new infrastrucre we need to remove the code:

tags:
  - docker

In that way, the jobs will land on the new generic runners.

This is the doc about the new infrastructure: https://gitlab.docs.cern.ch/docs/Build%20your%20application/CI-CD/Runners/k8s-shared-runners/

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NOTE We start with the generic runners, to see if they work fine for our use cases. If not, we'll ask for more specific runners.