Docker.md: add docker-compose instructions

[neheb]

No description provided.

[hmoffatt]

The docker-compose instructions refer to a named volume called gerbera-config, which is not defined in the instructions.

[neheb]

doesn't it get automatically created?

[hmoffatt]

No:

ERROR: Named volume "gerbera-config:/var/run/gerbera:rw" is used in service "gerbera" but no declaration was found in the volumes section.

[hmoffatt]

You need to declare a volume with:

volumes:
  gerbera-config: {}