Allow user and group mapping

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Problem Description

I'm running the AdguardHome docker image, with the `conf` and `work` dirs mapped to local folders. Adguard creates files within these folders as root. I would like the option to map to another user, the way linuxserver.io images do: https://docs.linuxserver.io/general/understanding-puid-and-pgid

### Proposed Solution

Allow for `PUID` and `PGID` environment variables to be set, so that files written by AdguardHome will map to the user I specify, so that I don't run into permission issues.

### Alternatives Considered

Another option would be for me to use docker's built in `--user` flag:

https://docs.docker.com/engine/reference/run/

But for that to work the processes within the container would have to be able to cope with running as a different user, and not as root. I'm not sure if that's currently the case, and did not see anything in the documentation about that.

edit: By the way, my compliments on the project, really nice to use and works like a charm!