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
```
on:
  pull_request: {}
  push:
    branches:
    - main
    - master
    paths:
    - .github/workflows/semgrep.yml
  schedule:
  # random HH:MM to avoid a load spike on GitHub Actions at 00:00
  - cron: 44 6 * * *
name: Semgrep
jobs:
  semgrep:
    name: Scan
    runs-on: ubuntu-20.04
    env:
      SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
    container:
      image: returntocorp/semgrep
    steps:
    - uses: actions/checkout@v4
    - run: semgrep ci

on:
  pull_request: {}
  push:
    branches:
    - main
    - master
    paths:
    - .github/workflows/semgrep.yml
  schedule:
  # random HH:MM to avoid a load spike on GitHub Actions at 00:00
  - cron: 44 6 * * *
name: Semgrep
jobs:
  semgrep:
    name: Scan
    runs-on: ubuntu-20.04
    env:
      SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
    container:
      image: returntocorp/semgrep
    steps:
    - uses: actions/checkout@v4
    - run: semgrep ci
