feature request: support PUID + PGID environment settings

[kinslayer1982]

Hey,

I think it would be really nice to allow using the PUID and PGID environment setting for the container. This should eliminate many possible permission issues when using bind mounts instead of docker volumes.
A good reason for bind mounts (in my opinion) is that InvoiceShelf offers to save a copy of every pdf on disk. This makes it possible to easily retrieve invoices etc. even if the is a problem with the container or to make additional backups.

[Wobak]

Agreed, had to switch to docker volumes because of Cache path issues at startup when installing, and I would greatly appreciate at lease a bind mount destination for exporting all PDFs or running the app in a bind mount :)