Make Memcached work for tests under docker

[nephila-nacrea]

I run tests using docker (docker/compose-dev run fixmystreet script/test), but tests that hit Memcached (such as 'test waste max-per-day' in t/cobrand/bromley.t) are always skipped; if I force them not to be skipped, they fail. Is there a way to get Memcached working for tests under Docker?