feat: BASE_IMAGE build arg

Merge pull request #13 from paullessing/patch-1

Use command in docker entrypoint to inherit PID 1

Update README.md

Add STEAM_EPOCH argument

Remove ~/Mods before symlinking

Add steam beta build arg; Add proper arg doc to readme

Move args around for better build caching

Change quickstart command

Re-add config copying method since it's needed at runtime :( , more slight reordering

Reorder steps to do as much as possible in the build as the unprivileged user to reduce build time due to chowns

Merge branch 'master' of https://github.com/FragSoc/barotrauma-docker

Dockerfile cleanup, remove need for entrypoint script

README amendments

Attempt to improve mod install script