Fix example postgres docker-compose setup in docs

[fleshgolem]

Add a local volume to postgres container so changes get persisted between restarts

What type of PR is this?

• documentation

What this PR does / why we need it:

As it turned out on the discord yesterday, the example docker-compose.yml for a postgres setup is actually fairly misleading which can easily lead to frustrations: The postgres container does not get any volume attached, so it will never persist any data between restarts.

This quick change should alleviate that issue for inexperienced users (even though they probably shouldnt use postgres to begin with)

Which issue(s) this PR fixes:

AFAIK no issue has been filed for this

Release Notes

* Improved sample postgres setup in installation docs

[hay-kot]

The postgres container does not get any volume attached, so it will never persist any data between restarts.

Postgres uses named volumes defined in the Dockerfile so the data should be persistent by default without an explicit declaration. Though I think this is a good addition since explicit is better than relying implicit behavior.

Thanks for the PR!