feat(ci): Build Docker image via Nix

Signed-off-by: Steffen Vogel <steffen.vogel@opal-rt.com>

.gitlab-ci.yml

@@ -114,8 +114,22 @@ build:nix:
 
   script:
     - nix build --print-build-logs .
-    - nix bundle --out-link villas .
-    - cp -L villas artifacts/villas
+
+    - | # Build ARX bundle
+      nix bundle --out-link villas .
+      cp -L villas artifacts/villas
+
+    - | # Build Docker image
+      nix run nixpkgs#skopeo -- login \
+        --username ${CI_REGISTRY_USER} \
+        --password ${CI_REGISTRY_PASSWORD} \
+        ${CI_REGISTRY}
+
+      nix build '.#dockerImage'
+
+      nix run nixpkgs#skopeo -- copy --insecure-policy \
+        "docker-archive:./result" \
+        "docker://${DOCKER_IMAGE}:${DOCKER_TAG}-nix"
 
   # after_script:
   #   - | # Push whole store to cache

flake.nix

@@ -63,6 +63,13 @@
           withAllHooks = true;
           withAllNodes = true;
         };
+
+        dockerImage = pkgs.dockerTools.buildLayeredImage {
+          name = "villas-node";
+          tag = "latest-nix";
+          contents = [ villas-node ];
+          config.ENTRYPOINT = "/bin/villas";
+        };
       };
     in
     {