remove hard-coded image owner

ci-cd/github-actions.md

@@ -98,7 +98,7 @@ Now, we can add the steps required. The first one checks-out our repository unde
           context: ./
           file: ./Dockerfile
           push: true
-          tags: ushamandya/simplewhale:latest
+          tags: ${{ secrets.DOCKER_HUB_USERNAME }}/simplewhale:latest
 
       - name: Image digest
         run: echo ${{ steps.docker_build.outputs.digest }}
@@ -151,7 +151,7 @@ Using the cache we set up earlier for it to store to and to retrieve
           file: ./Dockerfile
           builder: ${{ steps.buildx.outputs.name }}
           push: true
-          tags:  ushamandya/simplewhale:latest
+          tags:  ${{ secrets.DOCKER_HUB_USERNAME }}/simplewhale:latest
           cache-from: type=local,src=/tmp/.buildx-cache
           cache-to: type=local,dest=/tmp/.buildx-cache
       - name: Image digest