Add some comments to the .gitlab-ci.yml file explaining how it works.

Re-name the "test" job to "lint" since we may actually have real tests
for the Ansible roles in future.

Remove the unneeded docker service.

Merge before_script and script together. before_script should be used
when it is likely a derived job will want to override it.

Instead of starting from our shared Python image, start from our Ansible
image. This has the advantage of having all of the required dependencies
for ansible-lint pre-installed so running the test job is quicker. This
reduces the average CI pipeline run from around 2:30 to 20 seconds.

GitLab CI jobs are already passed to the shell so having the 'bash -c
"..."' line in the job script is unnecessary.

Instead of searching for all files which match '*.y*ml' (which includes
files such as 'foo.y-not-a-role-ml') and which may catch YAML files
which are part of roles, pass ansible-lint the role directory instead.
Role directories are assumed to be any directory directly under the
'roles' directory in the repository root.

Rather than hiding the ansible-lint configuration in the command line in
the .gitlab-ci.yml file, make use of the .ansible-lint configuration
file to be a little more explicit.