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
```
collective: celery
tiers:
    - tiers: '*'
      labels: ['Backer ❤️']
      message: 'Hey <author>. Thank you for supporting the project!:heart:'
    - tiers: ['Basic Sponsor', 'Sponsor', 'Silver Sponsor', 'Gold Sponsor']
      labels: ['Sponsor ❤️']
      message: |
        Thank you for sponsoring the project!:heart::heart::heart:
        Resolving this issue is one of our top priorities.
        One of @celery/core-developers will triage it shortly.
invitation: |
  Hey <author> :wave:,
  Thank you for opening an issue. We will get back to you as soon as we can.
  Also, check out our [Open Collective](<link>) and consider backing us - every little helps!

  We also offer priority support for our sponsors.
  If you require immediate assistance please consider sponsoring us.

collective: celery
tiers:
    - tiers: '*'
      labels: ['Backer ❤️']
      message: 'Hey <author>. Thank you for supporting the project!:heart:'
    - tiers: ['Basic Sponsor', 'Sponsor', 'Silver Sponsor', 'Gold Sponsor']
      labels: ['Sponsor ❤️']
      message: |
        Thank you for sponsoring the project!:heart::heart::heart:
        Resolving this issue is one of our top priorities.
        One of @celery/core-developers will triage it shortly.
invitation: |
  Hey <author> :wave:,
  Thank you for opening an issue. We will get back to you as soon as we can.
  Also, check out our [Open Collective](<link>) and consider backing us - every little helps!

  We also offer priority support for our sponsors.
  If you require immediate assistance please consider sponsoring us.
