Upload GRR image to GitHub container registry instead of DockerHub. (#1026)

Author: mbushkov

.github/workflows/build.yml

@@ -4,7 +4,7 @@ env:
   GCS_BUCKET: autobuilds.grr-response.com
   GCS_BUCKET_OPENAPI: autobuilds-grr-openapi
   GCS_LATEST_PATH: _latest_server_deb
-  DOCKER_REPOSITORY: grrdocker/grr
+  DOCKER_REPOSITORY: ghcr.io/google/grr
 jobs:
 
   test-ubuntu:
@@ -259,13 +259,14 @@ jobs:
           export BRANCH=$(echo $GITHUB_REF | cut -d'/' -f 3)
           ./appveyor/docker_build/build_docker_image.sh
       - if: ${{ github.event_name == 'push' }}
-        name: Log in to Docker Hub
+        name: Login to GitHub Container registry
         uses: docker/login-action@v2
         with:
-          username: ${{ secrets.DOCKER_USERNAME }}
-          password: ${{ secrets.DOCKER_PASSWORD }}
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
       - if: ${{ github.event_name == 'push' }}
-        name: Push to ${{ env.DOCKER_REPOSITORY }}
+        name: Push to GitHub Container registry
         run: |
           docker push -a ${{ env.DOCKER_REPOSITORY }}
   upload: