Update deploy.yml

.github/workflows/deploy.yml

@@ -1,10 +1,19 @@
 name: Build & Deploy
-on: push
+
+on:
+  push:
+    branches:
+    - master
 
 jobs:
   deploy:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
+    - uses: actions/checkout@master
+    - name: Build Hugo site
+      uses: peaceiris/actions-hugo@v0.57.2
+      with:
+        args: --gc -b "/"
     - name: Purge cache
       uses: jakejarvis/cloudflare-purge-action@master
       env: