Fix and update the docs.ovh.com dev environment

[cortexx]

• Update the Dockerfile to add missing sub-modules related to the docs-rendering repository which breaks the Docker image build
• Update the Dockerfile to bump the used Python version to prevent deprecation warnings
• Rework the Dockerfile and add some labels
• Update the documentation and implementation of the markdown linter's recommendations
• Rework the entrypoint script to apply some shell-check linter's recommendations