Updates to the directory structure, and other small additional tweaks

[FlorentLM]

• Removed the builder step by building directly on Steam's Ubuntu image, leading in a smaller final container (efficiency score 95% according to dive Actually added the builder back, but after reorganising and cleaning up some stuff, image is now only 718 Mb with 99% efficiency.
• Removed the need to reacquire root during the build by directly setting the correct permissions via --chown and --chmod flags in the Dockerfile
• Added support for build arg TMOD_VERSION=latest so it automatically grabs the latest available release
• Reworked the directory structure like so:
  1. • The server itself lives in /terraria-server
  2. • Worlds, Mods, server_config.txt and enabled_mods.json all live in /data This means users can now bind mount one folder on the host to /data and they'll find everything in there, which is useful for persistance across restarts.

NOTE: As per the recommended approach, users must chown the host folder to 456:456 before starting the container, otherwise the container might not be able to write its stuff in it.

Example:

chown -R 456:456 /path/on/the/host/terraria
docker run -v /path/on/the/host/terraria:/data 

[JACOBSMILE]

These were included in a different PR. Closing.