New make command: `docker-docs`.
like `make docs` just inside a container.

Also added CI workflow to test it.
```
#14 exporting to image
#14 exporting layers
#14 exporting layers 9.7s done
#14 writing image sha256:a7b15054abc118eb349dc96b4c51d0471bc05bd94487373e7f15ef03dbe44175 done
#14 naming to docker.io/celery/docs done
#14 DONE 9.7s
Creating docker_docs_1 ... 
Creating docker_docs_1 ... done
Waiting 60 seconds for docs service to build the documentation inside the container...
build succeeded, 122 warnings.
Stopping docker_docs_1 ... 
Stopping docker_docs_1 ... done
Removing docker_docs_1 ... 
Removing docker_docs_1 ... done
Removing network docker_default
```
Added make docker-docs. New make command: `docker-docs`.
like `make docs` just inside a container.

Also added CI workflow to test it.
```
#14 exporting to image
#14 exporting layers
#14 exporting layers 9.7s done
#14 writing image sha256:a7b15054abc118eb349dc96b4c51d0471bc05bd94487373e7f15ef03dbe44175 done
#14 naming to docker.io/celery/docs done
#14 DONE 9.7s
Creating docker_docs_1 ... 
Creating docker_docs_1 ... done
Waiting 60 seconds for docs service to build the documentation inside the container...
build succeeded, 122 warnings.
Stopping docker_docs_1 ... 
Stopping docker_docs_1 ... done
Removing docker_docs_1 ... 
Removing docker_docs_1 ... done
Removing network docker_default
```
{"is_singleton": true}
{"is_singleton": true}