Add a docker service to the pre-commit job and appropriate variables to
allow pre-commit jobs to use docker images should they desire.

Since this change involves adding a `before_script`, future-proof
ourselves against jobs wanting to customise the `before_script` section
of the config by making `pre-commit` into a template job. This means
that jobs can *extend* `before_script` using `!reference` tags.

Closes #62