Set up a docker dev environment

[FjolleJagt]

It would be nice if docker could mount an external directory containing the sources as a volume. This means that one could alter the code and reload the page to see one's changes, rather than having to re-run docker-compose. The obvious way of doing this doesn't work, because it mounts with the wrong ownership (it mounts as 1000:1000, not www-data:www-data).