Add `docker-compose` for spinning up docker instances

[PGijsbers]

Development of the API may involve a number of docker instances:

• A MySQL database
• The new Python-based REST API
• The old PHP-based REST API (for testing purposes)

It would be good if they could be managed from a single docker-compose file.
See also the discussion in #71

[PGijsbers]

Added in #74