Try to use nixos host runner

.gitea/workflows/build.yaml

@@ -16,7 +16,7 @@ env:
 jobs:
   build-and-push:
     # We use the standard ubuntu-latest to build our custom runner
-    runs-on: ubuntu-latest 
+    runs-on: nixos
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4

Dockerfile

@@ -1,24 +1,34 @@
-# Start with a official gitea runner image based on ubuntu with node installed
+# Start with an official gitea runner image based on ubuntu with node installed
 FROM docker.gitea.com/runner-images:ubuntu-latest-slim
 
+# Explicitly set the USER environment variable (expected by the Nix installer)
+ENV USER=root
+
 # Install system dependencies
 RUN apt-get update && \
-    apt-get install -y sudo bash jq xz-utils curl && \
+    apt-get install -y sudo bash jq xz-utils curl git ca-certificates && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
 
-# Install Nix as a multi-user installation
-RUN curl -L https://nixos.org/nix/install | sh -s -- --daemon
-ENV PATH="/nix/var/nix/profiles/default/bin:$PATH"
-
-# Enable flakes and nix-command
+# Pre-configure Nix: 
+# 1. Enable flakes
+# 2. Disable build-users-group (Fixes the missing 'nixbld' group error for root installs)
+# 3. Add devenv substituters for fast compiling
 RUN mkdir -p /etc/nix && \
-    echo "experimental-features = nix-command flakes" >> /etc/nix/nix.conf
+    echo "experimental-features = nix-command flakes" > /etc/nix/nix.conf && \
+    echo "build-users-group =" >> /etc/nix/nix.conf && \
+    echo "extra-substituters = https://devenv.cachix.org" >> /etc/nix/nix.conf && \
+    echo "extra-trusted-public-keys = devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw=" >> /etc/nix/nix.conf && \
+    echo "trusted-users = root" >> /etc/nix/nix.conf
 
-# Install devenv (the Nix way, not the apt-get way)
-RUN nix profile add nixpkgs#devenv
+# Install Nix in single-user mode (--no-daemon)
+RUN curl -L https://nixos.org/nix/install | sh -s -- --no-daemon
 
-# Cleanup, otherwise nix panicks
-RUN rm -rf /homeless-shelter
+# Update PATH so subsequent RUN commands and the final container can find Nix
+ENV PATH="/root/.nix-profile/bin:/nix/var/nix/profiles/default/bin:$PATH"
+
+# Install devenv using the modern flake URL and clean up build garbage
+RUN nix profile install --accept-flake-config github:cachix/devenv/latest && \
+    nix-collect-garbage -d
 
 CMD ["/bin/bash"]