the docker workflow chained off the backend workflow only, and backend has a
backend/** path filter. master pushes touching just frontend/apps or the docker
files never triggered docker.yml, so no master image was published and
remark42.com was not redeployed. broken since the build workflow was split in
#1977.
listen to workflow_run from both backend and frontend, and add Dockerfile,
docker-init.sh and .dockerignore to the backend workflow paths to restore the
path coverage the old build workflow had.