Drastically simplify the defaults and the README

[jbaiter]

So I've been setting up a Docker setup for Hymir for the past few hours.
Some things I noticed:

• The defaults are way too far-reaching, we're creating files in /var/log and /local, this is really unhygienic
• The main culprits are the default logging config, Java Melody and the Tomcat Access log.
• The logging defaults should only log to stdout
• Is Java Melody even needed with all the Spring Boot Management stuff?
• The Tomcat Access log should be activated by the user, not the defaults

Also, the README is a bit messy. It would be better to keep it short and simple and move all the advanced stuff into a separate documentation page (e.g. created with mkdocs).