CI: Actually enable the default fallback behavior of docker-pushrm

The Github action explicitly sets `README.md` as the default file to
push, see
https://github.com/christian-korneck/update-container-description-action/blob/master/action.yml#L17

This disables the fallback to `README-containers.md`, as implemented
in the actual tool that the Github action uses, i.e.
https://github.com/christian-korneck/docker-pushrm

However, by setting the file name explicitly to an empty string, we
can trigger the default fallback behavior of dockre-pushrm after all.

Signed-off-by: beorn7 <beorn@grafana.com>

.github/workflows/container_description.yml

@@ -30,6 +30,9 @@ jobs:
           destination_container_repo: ${{ env.DOCKER_REPO_NAME }}
           provider: dockerhub
           short_description: ${{ env.DOCKER_REPO_NAME }}
+          # Empty string results in README-containers.md being pushed if it
+          # exists. Otherwise, README.md is pushed.
+          readme_file: ''
 
   PushQuayIoReadme:
     runs-on: ubuntu-latest
@@ -49,3 +52,6 @@ jobs:
         with:
           destination_container_repo: ${{ env.DOCKER_REPO_NAME }}
           provider: quay
+          # Empty string results in README-containers.md being pushed if it
+          # exists. Otherwise, README.md is pushed.
+          readme_file: ''