Print checksum to job summary (#589)

Makes it easier to copy out for making PR to Docker image repo

.github/workflows/build.yml

@@ -95,7 +95,11 @@ jobs:
           mv Northstar.release.${{ env.NORTHSTAR_VERSION }}.zip ../
       - name: Compute SHA-512 checksum
         run: |
-          sha512sum Northstar.release.${{ env.NORTHSTAR_VERSION }}.zip
+          ZIP_CHECKSUM=$(sha512sum Northstar.release.${{ env.NORTHSTAR_VERSION }}.zip)
+          echo $ZIP_CHECKSUM
+          echo "```" >> $GITHUB_STEP_SUMMARY
+          echo $ZIP_CHECKSUM >> $GITHUB_STEP_SUMMARY
+          echo "```" >> $GITHUB_STEP_SUMMARY
       - name: Upload zip to release draft
         uses: softprops/action-gh-release@v1
         if: startsWith(github.ref, 'refs/tags/v') && !contains(env.NORTHSTAR_VERSION, '-rc')