ci: allow build on pull requests

Author: scribam

.github/workflows/docker.yml

@@ -1,6 +1,9 @@
 name: CI-Docker
 
 on:
+  pull_request:
+    branches:
+      - master
   push:
     branches:
       - master
@@ -27,8 +30,9 @@ jobs:
       with:
         username: ${{ secrets.DOCKER_USERNAME }}
         password: ${{ secrets.DOCKER_PASSWORD }}
+      if: github.event_name != 'pull_request'
 
     - uses: docker/build-push-action@v3
       with:
-        push: true
+        push: ${{ github.event_name != 'pull_request' }}
         tags: vitasdk/buildscripts:latest