Add Nomad deployment configuration and CI/CD pipeline

- Add docker.nix for Nix-based Docker image builds
- Update flake.nix with dockerImage package output
- Add output: standalone to next.config.ts for production builds
- Add /metrics endpoint for Prometheus scraping
- Add Gitea Actions workflow calling shared deploy-nomad.yaml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -1,5 +1,5 @@
-# ABOUTME: Nix flake for PhaseFlow development environment.
-# ABOUTME: Provides Node.js 24, pnpm, turbo, lefthook, and Ralph sandbox shell.
+# ABOUTME: Nix flake for PhaseFlow development environment and Docker build.
+# ABOUTME: Provides Node.js 24, pnpm, turbo, lefthook, and Docker image output.
 {
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
 
@@ -16,6 +16,12 @@
         pocketbase
       ];
     in {
+      # Docker image for production deployment
+      packages.${system} = {
+        dockerImage = import ./docker.nix { inherit pkgs; };
+        default = import ./docker.nix { inherit pkgs; };
+      };
+
       devShells.${system} = {
         # Default development shell with all tools
         default = pkgs.mkShell {