`docker-compose.yml` for `postgresql` specifies a local volume that ends up being owned by `root` on the host

Disclaimer: I'm not a docker superuser, especially when it comes to managing permissions and users between the host and containers, so it's very possible I'm doing something wrong.

Doing a standard generation with `--compose --postgresql` seems to generate a `docker-compose.yml` that specifies a volume mapping of `./tmp/db:/var/lib/postgresql/data`; the resulting `tmp/db` directory is owned by `root` on the host system which is problematic - this caught me out specifically because when I attempted to do `docker build .` it would fail since it wasn't allowed to read that file.

I suspect this is probably not a direct bug in the gem itself, but it's the sort of thing I feel this gem should be trying to help with.

(I assume this probably happened with all variations that configure volumes, but have only used `postgresql` myself)