Update git command in build docker image workflow (#278)

* empty

* replace by head

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
 
       - name: Add SHORT_SHA env property with commit short sha
         run: |
-          SHORT_SHA=$(git rev-parse --short=9 ${{ github.event.pull_request.head.sha }})
+          SHORT_SHA=$(git rev-parse --short=9 HEAD)
           echo "SHORT_SHA=$SHORT_SHA" >> $GITHUB_ENV
           echo "Short SHA: $SHORT_SHA"