Enable Docker build on PRs for CI validation

The CI workflow now builds the Docker image on pull requests to 
catch build issues early, but only pushes the image to Docker Hub 
on main branch updates. This ensures Docker builds are tested for 
all PRs without publishing images from unmerged code.

Author: rwest

.github/workflows/CI.yml

@@ -400,7 +400,10 @@ jobs:
     # taken from https://github.com/docker/build-push-action
     needs: [build-and-test, regression-test]
     runs-on: ubuntu-latest
-    if: github.ref == 'refs/heads/main' && github.repository == 'ReactionMechanismGenerator/RMG-Py'
+    # Run on push to main or on pull requests
+    if: |
+      (github.ref == 'refs/heads/main' && github.repository == 'ReactionMechanismGenerator/RMG-Py') ||
+      (github.event_name == 'pull_request')
     steps:
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
@@ -409,15 +412,16 @@ jobs:
         uses: docker/setup-buildx-action@v3
 
       - name: Login to Docker Hub
+        if: github.ref == 'refs/heads/main' && github.repository == 'ReactionMechanismGenerator/RMG-Py'
         uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
-      - name: Build and Push
+      - name: Build (and Push if on main)
         uses: docker/build-push-action@v6
         with:
-          push: true
+          push: ${{ github.ref == 'refs/heads/main' && github.repository == 'ReactionMechanismGenerator/RMG-Py' }}
           tags: reactionmechanismgenerator/rmg:latest
           build-args: |
             RMS_Branch=for_rmg