ci: add woodpecker workflow for building frontend

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

.woodpecker/frontend.yaml

@@ -0,0 +1,19 @@
+---
+when:
+  - event: push
+  - event: pull_request
+    path:
+      - 'frontend/**'
+      - flake.nix
+      - flake.lock
+      - VERSION
+
+steps:
+  - name: build
+    image: docker.io/nixos/nix
+    environment:
+      NIX_CONFIG: 'experimental-features = nix-command flakes'
+    commands:
+      - nix -vL build .#frontend
+      - mkdir ./dist
+      - cp -RL ./result/* ./dist/