use volumes instead of bind mounts

[ollyg]

mkdir -p netdisco/{logs,config,nd-site-local} should probably be mkdir -p netdisco/{logs,config,nd-site-local,pgdata}

[rc9000]

Actually this doesn't change much, since the container will "help" on startup:

netdisco-postgresql_1  | fixing permissions on existing directory /var/lib/postgresql/data ... ok

So the only difference is that with the mkdir, I now have a 70:<my primary group> instead of a 70:root directory, but both aren't group readable anyways :)

ram@cicd:/tmp/issue52 $ ls -ld /tmp/no_mkdir/netdisco/pgdata /tmp/si_mkdir/netdisco/pgdata
drwx------ 19 70 root     4096 Feb  3 01:27 /tmp/no_mkdir/netdisco/pgdata
drwx------ 19 70 ram      4096 Feb  3 01:40 /tmp/si_mkdir/netdisco/pgdata

I suspect that a better solution to all this permission/mkdir micromanagement would be to use volumes instead of bind mounts. But I'm a bit of a troglodyte when it comes to this corner of docker, we'd probably need to look at some other projects with a similar hodgepodge of users, storage areas, config files and containers to learn how to do this properly.

I'd also expect a lot of documentation and support friction from that, so it's probably something we should only do with severe breaking changes anyways, like next time we update the Postgres major version.