Build a cross-platform devenv.sh nix-shell, and container

[hh]

It should minimally have emacs from HEAD (feel free to pin) and our doom config loaded.

Tiny successes along the way:

• nix to OCI image (just make it work minimally)
• nix with emacs (from HEAD) to OCI image
• nix with emacs + doom + ii/doom-config to OCI (fine to override the location so it's not in HOME if necessary)
• nix with emacs + doom + ii/doom-config + broadwayd (so we can run emacs as a web page)

Removing https://devenv.sh/containers/ dependency as it may be too heavy.

[hh]

@zachmandeville let's get a walk-thru of this today. I'd like to see where we are at.

[hh]

Compile options for emacs:
https://github.com/cloudnative-coop/coop-templates/tree/main/iidoom

[zachmandeville]

am developing this at ii/doomnix in the try2 pr. Would love to pair first chance for an hour or so to help share knowledge and get help on this as I am having a difficult time.