Wrong advice in AiO -> Multi Container Migration guide about database

Reported by @AMoonRabbit here:

The second issue was that I had DATABASE_URL variables set when for docker installs these are not required. After commenting these lines out > I no longer had issues e.g. permission denied for table django_migrations or user role funkwhale does not exist.

So you could probably adapt the existing migration article with a few changes?

I am not sure, though, if this is already true for 1.2.0, but it is for sure in 1.3.0 so we should adjust.