Adding RTD prefix for docker only in setting.py and all other places where is needed

[Iamshankhadeep]

fixes #5586

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Iamshankhadeep]

@stsewd can you review it?

[stsewd]

Thanks, there are still some places to update

https://github.com/readthedocs/readthedocs.org/blob/master/docs/development/buildenvironments.rst
https://github.com/readthedocs/readthedocs.org/blob/master/readthedocs/settings/dev.py

readthedocs.org/readthedocs/config/config.py

Line 282 in 0034c1a

Returns supported versions for the ``DOCKER_DEFAULT_VERSION`` if not

readthedocs.org/readthedocs/projects/tasks.py

Line 374 in 0034c1a

``settings.DOCKER_ENABLE`` is used)

readthedocs.org/readthedocs/doc_builder/environments.py

Line 776 in 0034c1a

If :py:data:`settings.DOCKER_ENABLE` is true, build documentation inside a

[stsewd]

Thanks!

[Iamshankhadeep]

Nice 2 work done. now lets focus on new issues.