Use a different sha for the pull request flow

.github/workflows/docker.yml

@@ -68,7 +68,14 @@ jobs:
         run: echo "TAG=${GITHUB_REF/refs\/tags\//}" >> $GITHUB_ENV
 
       - name: Add SHORT_SHA env property with commit short sha
-        run: echo "SHORT_SHA=$(echo ${GITHUB_SHA} | cut -c1-8)" >> $GITHUB_ENV
+        run: |
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
+            SHA="${{ github.event.pull_request.head.sha }}"
+          else
+            SHA="${GITHUB_SHA}"
+          fi
+          echo "SHORT_SHA=${SHA:0:8}" >> $GITHUB_ENV
+
 
       - name: Login to Docker for building
         run: echo "${{ secrets.DOCKER_PASSWORD }}" | docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin