Should have mentioned I've ran up the image with docker run and the directory didn't exist
Edit 2.
This is also about 23 minutes into the build when it throws this error; after having built a lot of packages and derivations already. I'm also using cachix/nix-install-action to get nix on the running container.
Edit 3.
Finally got it building this morning, I haven't dug into it but switching from the cachix/nix-install-action to manually installing the determinate systems nix installer (action didn't work for a strange reason).
Either pick a build image that doesn't have /homeless-shelter, like nixos/nix, or remove it with something like:
# UNTESTED
RUN rmdir /homeless-shelter
The root cause is that your build filesystem is dirty. When Nix sandboxes a build, it runs the builders as nobody, a permissionless user with no home directory. On Linux, users with no home directory get their $HOME set to /homeless-shelter, and Nix relies on this directory not existing.