Allow running docker images as non-root user

[tribut]

This changes permissions inside the docker image in order to be able to run linkwarden without root permissions.

I decided against creating a special-purpose user as proposed in #701, because this would be a breaking change for existing installs (requires changes to ownership of existing bind-mounts). Instead, I've added configuration to docker-compose.yml that new installs will pick up and allows customization by the user.

Fixes #550 Fixes #655 Fixes #712

[CLAassistant]

All committers have signed the CLA.

[daniel31x13]

Hey thanks for the contribution! Gonna merge this soon.

[daniel31x13]

Just took a look at this, there are two major issues after running this:

1. Well, it doesn't run - keeps giving me an error that it can't reach the database:

2. The size of the container increased to 5.37GB:

[discotimetraveler]

Getting this working is critical. There's a lot of helpful info over here. https://github.com/linkwarden/linkwarden/issues/799

[tribut]

Thanks for taking a look, Daniel. Don't have much free time currently, but will keep this on my list.

If anybody else needs this urgently, feel free to take a look yourselves. I am sure I tried this, so my best guess is the problem is related to permissions of mounted volumes.

[ed0mm]

Hi! I'm confused if the problems with preserved formats are still tied to this or rather things discussed here? And are these commits still problematic or could they be merged?

[holocronweaver]

I made an updated version of this PR here, fixes the Docker image size issue: #1483