[Feature Request] Publicize Holo container image referenced in steamos-devmode

[matte-schwartz]

I've been doing more development work and testing on my Steam Deck lately, and the steamos-devmode script has been an invaluable tool when I need to build and test packages like gamescope on SteamOS. The script explicitly recommends:

SteamOS Developer Mode

Important: This will allow potentially breaking changes to the root filesystem.
  This is meant for developers and technical users who know what they are doing.
  Changes to the root filesystem will be overwritten by the next SteamOS update.

Developers:
  - Consider packaging your application with flatpak, rather than
    invoking/requiring this script.  This is a much better (and safer) experience
    for users
  - Consider building your package in the Holo container images with
    distrobox/toolbox

yet as far as I can tell, this Holo container image does not exist anywhere publicly. I assume one has to exist given this wording. Am I missing a link to it somewhere, or is this referencing some sort of third-party Holo container image? Thanks!