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
```
version: '3'

services:
  celery:
    build:
      context: ..
      dockerfile: docker/Dockerfile
      args:
        CELERY_USER: developer
    image: celery/celery:dev
    environment:
      TEST_BROKER: pyamqp://rabbit:5672
      TEST_BACKEND: redis://redis
      PYTHONUNBUFFERED: 1
      PYTHONDONTWRITEBYTECODE: 1
      REDIS_HOST: redis
      WORKER_LOGLEVEL: DEBUG
      AZUREBLOCKBLOB_URL: azureblockblob://DefaultEndpointsProtocol=http;AccountName=devstoreaccount1;AccountKey=Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==;BlobEndpoint=http://azurite:10000/devstoreaccount1;
      PYTHONPATH: /home/developer/celery
    tty: true
    volumes:
      - ../.:/home/developer/celery
    depends_on:
      - rabbit
      - redis
      - dynamodb
      - azurite

  rabbit:
    image: rabbitmq:latest

  redis:
    image: redis:latest

  dynamodb:
    image: amazon/dynamodb-local:latest

  azurite:
    image: mcr.microsoft.com/azure-storage/azurite:latest

  docs:
    image: celery/docs
    build:
      context: ..
      dockerfile: docker/docs/Dockerfile
    volumes:
        - ../docs:/docs:z
    ports:
      - "7000:7000"
    command: /start-docs
version: '3'

services:
  celery:
    build:
      context: ..
      dockerfile: docker/Dockerfile
      args:
        CELERY_USER: developer
    image: celery/celery:dev
    environment:
      TEST_BROKER: pyamqp://rabbit:5672
      TEST_BACKEND: redis://redis
      PYTHONUNBUFFERED: 1
      PYTHONDONTWRITEBYTECODE: 1
      REDIS_HOST: redis
      WORKER_LOGLEVEL: DEBUG
      AZUREBLOCKBLOB_URL: azureblockblob://DefaultEndpointsProtocol=http;AccountName=devstoreaccount1;AccountKey=Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==;BlobEndpoint=http://azurite:10000/devstoreaccount1;
      PYTHONPATH: /home/developer/celery
    tty: true
    volumes:
      - ../.:/home/developer/celery
    depends_on:
      - rabbit
      - redis
      - dynamodb
      - azurite

  rabbit:
    image: rabbitmq:latest

  redis:
    image: redis:latest

  dynamodb:
    image: amazon/dynamodb-local:latest

  azurite:
    image: mcr.microsoft.com/azure-storage/azurite:latest

  docs:
    image: celery/docs
    build:
      context: ..
      dockerfile: docker/docs/Dockerfile
    volumes:
        - ../docs:/docs:z
    ports:
      - "7001:7000"
    command: /start-docs