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 :)

[duckfullstop]

Sorry for the horrendous delay here, life happened 🙇

I'm planning on reviewing this in the coming days when I'm a bit more awake, but my personal preference is to make the container run properly using an external UID/GID override (eg --USER 1001:65535, the docker-compose user: 1001:65535 flags, or Kubernetes's PodSecurityPolicy) for a more standards-driven approach to this problem. This, off the top of my head, should be trivial so long as the entrypoint and main binary are executable by all. (data files only need to be readable by all).

I'm not averse to additionally merging this to provide a simple envvar based way of handling the issue, but is there a strong argument to do so over simply being tolerant to any uid/gid combination running?

(Update: I have tested with the --USER flag in podman and all functionality seems to be working properly - can someone else verify for me? See #14)

[duckfullstop]

Code itself LGTM though.

[flexd]

docker-compose files usually use environment variables for UID/PID as far as I know.
I'm mounting a folder into the containers from the host system, so I don't want to be changing the ownership or permissions on those files to whatever the container happenst to have internally. The solution in this PR of adopting the uid/gid that is specified for the openttd makes it so that it just works regardless.
I don't have any experience with podman so I don't know how it handles things

What this PR does differently over just overriding UID/GID is that the openttd user has it's UID/GID changed to the specified one. We no longer need to chown any of those files or change the permissions to be accessible by all