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
```
name: Docker

on:
    pull_request:
      branches: [ 'main']
      paths:
          - '**.py'
          - '**.txt'
          - '**.toml'
          - '/docker/**'
          - '.github/workflows/docker.yml'
          - 'Dockerfile'
    push:
      branches: [ 'main']
      paths:
          - '**.py'
          - '**.txt'
          - '**.toml'
          - '/docker/**'
          - '.github/workflows/docker.yml'


jobs:
  docker-build:
    runs-on: ubuntu-latest
    timeout-minutes: 60
    steps:
    - uses: actions/checkout@v4
    - name: Build Docker container
      run: make docker-build
name: Docker

on:
    pull_request:
      branches: [ 'main']
      paths:
          - '**.py'
          - '**.txt'
          - '**.toml'
          - '/docker/**'
          - '.github/workflows/docker.yml'
          - 'Dockerfile'
    push:
      branches: [ 'main']
      paths:
          - '**.py'
          - '**.txt'
          - '**.toml'
          - '/docker/**'
          - '.github/workflows/docker.yml'


jobs:
  docker-build:
    runs-on: ubuntu-latest
    timeout-minutes: 60
    steps:
    - uses: actions/checkout@v4
    - name: Build Docker container
      run: make docker-build

  docker-docs:
    runs-on: ubuntu-latest
    timeout-minutes: 5
    steps:
    - uses: actions/checkout@v4
    - name: Build Documentation
      run: make docker-docs
