added docker volume for storing the postgres DB

[jstucke]

addresses the problems with lost database contents from issue #27

• creates a volume for storing the postgres DB data outside and independently of the postgres container so that replacing the container shouldn't lead to loss of analysis data

[maringuu]

If I understand correctly the problem is calling docker-compose down. From the [docs(https://docs.docker.com/compose/reference/)

down Stop and remove containers, networks, images, and volumes

Meaning this PR will not help.

Are there any other benefits of creating a volume? Are containers recreated automatically somewhen?

[maringuu]

Aside the comment this LGTM.

[maringuu]

Landed in 6f1c9b9189a13418b42159a128c29402a844ff3f