Enhance action.yml with docker-image input

Add docker-image input to Piper Maven build step

Author: Schmarvinius

.github/actions/build/action.yml

@@ -32,6 +32,7 @@ runs:
       uses: SAP/project-piper-action@main
       with:
         step-name: mavenBuild
+        docker-image: ''
 
     - name: Mutation Testing
       if: ${{ inputs.mutation-testing == 'true' }}