With the updated hooks, address some newly found issues:
- Sort Python imports with isort.
- Don't cache pip packages in the container image.

Use the git skeleton file.

Use the renamed CI templates repository.

If the message is an empty string or in the fallback case of using the
request data if the message is None, don't send a notification.

- Remove the manaul stages order, pre-commit now runs in .pre.
- Use a more specific name for building the container image.

There's an issue with version 5.0.0, it requires six but it's missing
from the package requirements. Pin to an older version until it's fixed.

Remove runamel.yaml, json output is available and better.

- Empty service (just the healthcheck).
- Docker image.
- Pre-commit.
- GitLab CI.