fix .github/workflows, no need to a pdocker proxy

Author: oliverkurth

.github/workflows/photon-os-installer.yml

@@ -16,7 +16,7 @@ jobs:
       run: |
         POI_IMAGE_NAME=photon/installer:${{ github.sha }}
         cd docker
-        docker build -t $POI_IMAGE_NAME --build-arg DOCKER_PROXY=dockerhub.artifactory.eng.vmware.com/ --build-context poi-helper=$(realpath $(pwd)/..) .
+        docker build -t $POI_IMAGE_NAME --build-context poi-helper=$(realpath $(pwd)/..) .
         docker system prune -f
 
   test: