feat: let renovate pin version of postgres used in CI and local dev

Add a `POSTGRES_DOCKER_IMAGE` variable which is used to specify the
postgres image to use in CI for running tests. This variable will be
respected both by CI jobs which spin up a GitLab CI service and pytest
spinning up its own container.

Teach renovate how to update this image.

For local development, specify the version of the postgres image to be
used. This then becomes visible to renovate for update.

Renovate recognises that the same postgres dependency is used and so
will naturally group updating `POSTGRES_DOCKER_IMAGE` and the use of
`image: postgres:...` in the `docker-compose.yml` file together.

The renovate configuration was tested locally in a [testing branch of
the ballots webapp][1].

[1]: https://gitlab.developers.cam.ac.uk/uis/devops/
  regent-house-ballots/ballots-webapp/-/commits/postgres-pin-experiment