Update docker build, add instructions to README

[bo0tzz]

Fixes #5, #6

[cachapa]

Thank you!

[cachapa]

BTW, what happens on upgrades, does the Postgres database get wiped?

Would it make sense to have some light documentation on how to store the data appropriately?

[bo0tzz]

The postgres data is stored in a docker volume called pgdata. That keeps existing across updates, reboots, etc unless the user explicitly removes it. An alternative is to change - pgdata:/var/lib/postgres/data to use a mounted folder, like - /some-folder:/var/lib/postgres/data.

[cachapa]

Thanks for the clarification 👍