Use .env for mopidy config

[whomwah]

This lets users use their .env for the mopidy config variables. Means they can set:

MOPIDY_USERNAME=<SPOTIFY username for Mopidy>
MOPIDY_PASSWORD=<SPOTIFY password for Mopidy>
MOPIDY_CLIENT_ID=<SPOTIFY client id for Mopidy>
MOPIDY_CLIENT_SECRET=<SPOTIFY client secret for Mopidy>

in their .env and it will be copied to the container when it starts up. I needed to remove the volume mount to do this. I think it's only these variable that a user it likely to need to change at the mo.

I've also changed mopidy.local to mopidy to potentially save any issues in the office with the .local domain. I had problems using this domain at home for some reason.

I've also added persistence so you can carry on where you left off.

[atothewest]

👍 looks great.