Make Dockerfile rebuild only on PR open and synchronize that edits Do…

…ckerfile

Previously Dockerfile would be rebuilt on PR open and any new commits to the PR, resulting in rebuilding/committing changes to the build scripts many times even if the Dockerfile doesn't change.

.github/workflows/update_build_environment.yml

@@ -1,6 +1,7 @@
 name: Rebuild and publish new ubcdsci/py-intro-to-ds image on DockerHub
 on:
   pull_request:
+    types: [opened, synchronize]
     paths:
       - Dockerfile
 jobs:
@@ -14,7 +15,24 @@ jobs:
       with:
         fetch-depth: '0'
         ref: ${{ github.head_ref }}
+    - name: Check if Dockerfile needs to be rebuilt
+      id: check-stale
+      run: |
+        if [[ ${{ github.event.action == 'opened' }} ]]; then
+            echo "stale_dockerfile=1" >> "$GITHUB_OUTPUT"
+            echo "PR just opened, and it edits the Dockerfile, so rebuilding the image."
+        else
+            git diff --quiet ${{ github.event.before }} ${{ github.event.after }} Dockerfile
+            echo "stale_dockerfile=$?" >> "$GITHUB_OUTPUT"
+            if [[ ${{ steps.check-stale.outputs.stale_dockerfile == 0 }} ]]; then
+                echo "PR synchronized, but Dockerfile was not edited. Not rebuilding the image."
+            else
+                echo "PR synchronized, and Dockerfile was edited, so rebuilding the image."
+            fi
+        fi
+    - name: Check if Dockerfile needs to be rebuilt
     - name: Rebuild and publish image
+      if: ${{ steps.check-stale.outputs.stale_dockerfile }}
       id: rebuild
       uses: elgohr/Publish-Docker-Github-Action@v5
       with:
@@ -24,6 +42,7 @@ jobs:
         dockerfile: Dockerfile
         snapshot: true
     - name: Update build_html.sh script
+      if: ${{ steps.check-stale.outputs.stale_dockerfile }}
       run: |
         git config --local user.email "[email protected]"
         git config --local user.name "GitHub Action"
@@ -33,6 +52,7 @@ jobs:
         git add build_html.sh
         git commit -m "update build_html.sh script with new docker image"
     - name: Update build_pdf.sh script
+      if: ${{ steps.check-stale.outputs.stale_dockerfile }}
       run: |
         git config --local user.email "[email protected]"
         git config --local user.name "GitHub Action"
@@ -42,6 +62,7 @@ jobs:
         git add build_pdf.sh
         git commit -m "update build_pdf.sh script with new docker image"
     - name: Push changes to build scripts
+      if: ${{ steps.check-stale.outputs.stale_dockerfile }}
       uses: ad-m/github-push-action@master
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}