Revisit container repository

Currently there are various PRs and issues piling up in the repository. The README has aged quite a lot in the past few years and could take a rewrite from scratch. Also we should check the assumptions the containers were build with compared to the state of the art.

Various steps have already took place thanks to @hugopeixoto, but we still have some work to do. I would be open to suggestions and people willing to take the challenge :)

Topics I would like to put on the table:

- [ ] Rewriting README, remove boilerplate content, maybe add a screenshot, link to the new docs and web page.
- [ ] Dropping the alpine image (it doesn't provide much advantage over the debian image, but is unnecessarily inconvenient to work with due to musl-lib and potentially undisclosed security issues)
- [ ] Rework local uploads. Various people voiced concerns over the default image upload location and the current way of handling local upload, is not great.
- [x] Adding a healthcheck
- [ ] Maybe add sqlite support

There are two things that are probably solved better upstream, but are relevant in the container, mostly:
- [ ] Database configuration by env-var without `CMD_DB_URL` https://github.com/hedgedoc/container/issues/114
- [ ] Rework database migrations to prevent them from failing https://github.com/hedgedoc/container/issues/157 -> probably backporting [the umzug PR](https://github.com/hedgedoc/hedgedoc/pull/384) would solve this