Update build.yml

.github/workflows/build.yml

@@ -1,6 +1,10 @@
 name: Build
 run-name: Build the game
-on: [push]
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -19,3 +23,6 @@ jobs:
       
       - name: build game
         run: env LEAN_ABORT_ON_PANIC=1 lake build
+
+      - name: Build the Docker image
+        run: docker build . --file Dockerfile --tag nng:$(date +%s)