Update Docker setup

[trixter-osec]

Slightly modernize the current Docker setup:

• Upgrade postgres to 18 (note that bind mount is dropped in favor of volumes, as due to Change PGDATA in 18+ to /var/lib/postgresql/MAJOR/docker docker-library/postgres#1259 we cannot re-use existing bind mount data anyway, so easiest migration path on local development setup is to export the old database, git pull, run the docker compose, and then import the database). Production will be updated shortly after this gets merged.
• Sometimes there's a race between db being ready and the db-init container running, therefore swap to rely on redis/db being healthy instead of just started
• Drop nginx container as afaik it's leftover from the previous prod deployment, one should just configure nginx on the host to proxy to 127.0.0.1:3000 if needed
• Document .env.example with examples so that you can just clone the repository and run it