run application as non-root user in docker container

[m90]

This models the Dockerfile after the advice given here https://github.com/hexops/dockerfile which seems to make a lot of sense.

Trouble with this approach is that it will require manual interaction by users that upgrade, so we might need to wait for a proper point in time to sneak this in.

[m90]

Changing file ownership in the container would work like this:

docker exec -u 0 <container_ref> chown offen:offen /var/opt/offen/offen.db

[m90]

A possible migration strategy for moving here might be:

1. Starting with v0.4.0 we make the non-root Docker image the default, providing instructions on how to migrate. This should only affect deployments that use the Docker / SQLite combo.
2. For the entire v0.4.x series we also provide a v0.4.x-root tag on Docker Hub that still uses the Docker Image running as root. This leaves users with a little more time to migrate while still keeping the installs up to date.
3. Starting with v0.5.0 the image running as root is dropped. Anyone who wants to upgrade will need to run the upgrade steps against their setups.