feat: postgres backup to filesystem

[ticoombs]

Problem

Documented backup solutions (here) are inefficient and cause all data from the backup to be piped via the docker console/output rather than directly to a file.

Outputting directly to a file results in atleast a 4x speedup of backups, and does not require docker console to proxy the whole database.

(to be merged after #209)

[Nutomic]

I think this would fit better in the documentation. Then it will also benefit people who dont use Ansible.

https://github.com/LemmyNet/lemmy-docs/blob/main/src/administration/backup_and_restore.md

[dessalines]

Agree, we could just link to that backup and restore page on this readme, if you think that'd be useful.