Allows us to run the Docker Daemon test

The mock backend will not receive any backend hits.

It might take a long while to pull the image.

The Daemon does not seem to return an `IdentityToken`, we can
just pass the same authentication object to the pull/push

Can be enabled locally by providing:

- `CI_REGISTRY_IMAGE`
- `CI_COMMIT_SHA`
- `DOCKER_HOST`

It is likely that the auth variables will also be needed:

- `DOCKERODE_USERNAME`
- `DOCKERODE_PASSWORD`
- `DOCKERODE_SERVER`

Explain how to configure the plugin via configuration variables
and/or environment variables

Allows simplification of repeated test harnesses

We need to test against a _real_ Docker backend