update docker-compose configuration

Port the ./compose.sh-style docker-compose configuration to this application and update the instructions on getting started. This commit does not alter the CI configuration.

This commit is a forerunner for making a Hydra version 1 compatible flavour of the application.

We experiment with a slightly nicer way to auto-create OAuth2 clients by having the configuration for each client in a JSON file within compose/oauth2-clients/. See the README in that directory for more information.

compose/oauth2-clients/README.md

+## OAuth2 Client Configuration
+
+This directory holds Hydra resources representing client configurations which
+should be created when the Hydra server starts. They should be named
+``[id].json`` where ``[id]`` corresponds to the ``id`` field in the JSON body.