Support other repo owners in GH docker action

Being able to functionaly utilize the same GH Docker action can be
beneficial for users wanting to build, test and develop in their own
repos. To do so, use ${{ github.repository_owner }} in the action. That
should provide some rudimentary flexibility without breaking the
canonical repo.

Users of this functionality should take care to populate the required
secrets in Github

* DOCKERHUB_TOKEN, DOCKERHUB_USERNAME
* CR_PAT

.github/workflows/docker.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Generate Alpine Docker tag
         id: docker_alpine_tag
         run: |
-          DOCKER_IMAGE=miniflux/miniflux
+          DOCKER_IMAGE=${{ github.repository_owner }}/miniflux
           DOCKER_VERSION=dev
           if [ "${{ github.event_name }}" = "schedule" ]; then
           DOCKER_VERSION=nightly
@@ -32,7 +32,7 @@ jobs:
       - name: Generate Distroless Docker tag
         id: docker_distroless_tag
         run: |
-          DOCKER_IMAGE=miniflux/miniflux
+          DOCKER_IMAGE=${{ github.repository_owner }}/miniflux
           DOCKER_VERSION=dev-distroless
           if [ "${{ github.event_name }}" = "schedule" ]; then
           DOCKER_VERSION=nightly-distroless