Fix #14: allow for UID/GID specification via PUID/PGID env variables

[flexd]

Hi there!

I made it so you can specify PUID/PGID when starting this container, fixing #14.

TLDR; Add the openttd user as normal, but run a new entrypoint as root which changes the uid/gid on "openttd" to the specified ones and then drops privileges to that user and runs the original entrypoint

I took some inspiration from how LinuxServers do it [1] , and also this Dockerfile/entrypoint where gosu was used https://github.com/sudo-bmitch/jenkins-docker/tree/main

Using gosu seems to be common practice [3]

[1] https://github.com/linuxserver/docker-baseimage-ubuntu/blob/focal/root/etc/s6-overlay/s6-rc.d/init-adduser/run
[2] (https://github.com/tianon/gosu?tab=readme-ov-file#gosu
[3] https://denibertovic.com/posts/handling-permissions-with-docker-volumes/

[flexd]

@duckfullstop Mentioning you in case you didn't see this :)