feat(build): containerize ISO builds for non-pacman distros

this is a commit message. it is a MANIFESTO for no reason. i am aware.
it is 17:38. i am in pain. the pain is optional. the commit is not.

so. context, because i know you need it:
i installed gentoo to see what happens. what happened: emerge. days of
emerge. portage asked me to rebuild itself mid-rebuild and i said yes
because i was scared of what it'd do if i said no. my ext4 drive died
AGAIN so we're btrfs. snapshots are now my emergency contacts.
(profile update pending: "Used Gentoo BTW" -> "Used Gentoo and it
used me")

THE ACTUAL CHANGES (finally):
- artools is bash + pacman. it is not sacred. it is now a CONTAINER
- Containerfile: artix base, artools + squashfs-tools + xorriso +
  mkinitcpio, repo buildiso as ENTRYPOINT (the --overwrite='*' one.
  the other one knows what it did), antergos-pkgs pacman.conf baked in
- .dockerignore so podman users don't download 33MB of my .git and
  learn things about me

devtmpfs requires rootful podman. i am not explaining why. the answer
is "devtmpfs is a coward." i am the maintainer. i am allowed to say
that. rootless? you're not building an ISO, you're building a support
system for your decisions. anyway.

- build-iso-podman.sh exists so i can stop pasting commands from the
  README into my own terminal like a tourist
- README now has: podman for non-pacman (gentoo fedora debian), native
  for arch/artix/kaos, and a TABLE. i made a TABLE. i am healing.
- it works on any distro with podman because i want you to suffer LESS
  than me. that is not altruism. it is spite with a benefit package.

so laugh all you want. forget about me all you want. but at least i'll
be gorgeous forever. this is the longest i've committed to anything
since gentoo. and i regret BOTH of those decisions.

Signed-off-by: Michał <ash8820@proton.me>

.dockerignore

@@ -0,0 +1,4 @@
+.git
+.iso
+iso-output
+docs/_site