[GHA] Use secrets for dockerhub registry login

Author: elfiesmelfie

.github/workflows/build.yml

@@ -17,9 +17,7 @@ jobs:
         branch:
           - sid_amd64
     env:
-      # These three values need to be updated for collectd/collectd-ci
-      # SLUG: "collectd/ci:${{ matrix.branch }}"
-      SLUG: "ghcr.io/${{ github.actor }}/collectd-ci:${{ matrix.branch }}"
+      SLUG: "collectd/ci:${{ matrix.branch }}"
 
     steps:
     - uses: actions/checkout@v2
@@ -33,7 +31,6 @@ jobs:
     - name: Log into the container registry
       uses: docker/login-action@v1
       with:
-        registry: ghcr.io
-        username: ${{ github.actor }}
-        password: ${{ secrets.GITHUB_TOKEN }}
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
     - run: docker push "${SLUG}"