Cannot get recorder to connect to mariadb

I’ve just moved from running HA (0.115.4) on my own docker installation but using supervisor (I think it’s called docker supervised) to use pure hassio. I restored a snapshot and everything works OK except recorder.

I’m seeing the following in the log

homeassistant.components.recorder] Error during connection setup: (MySQLdb._exceptions.OperationalError) (2002, "Can't connect to MySQL server on 'core-mariadb' (115)")
(Background on this error at: http://sqlalche.me/e/13/e3q8) (retrying in 3 seconds)

This is repeated several times.

As far as I can see, the mariadb addon is working OK (version 2.2.1), but HA won’t connect. I’ve restarted everything several times.

Any ideas?