NOTE: If you happen to end up here 'by the hands of Google' or some other (mis)fortune - please have a look at the solution
I use Cesar William Alvarenga's setup
to get going - and I'm OK for the db, and for the build of phoenix and, heck, I'm even good for the docker-compose run --rm phoenix mix test
like this (my app is named battery_rehab - but otherwise it's a 1:1)
√ battery.rehab % docker-compose run --rm battery_rehab mix test
Creating batteryrehab_battery_rehab_run ... done
Compiling 17 files (.ex)