readthedocs: specify build-container

Docs builds currently fail with

Could not import extension sphinx.builders.linkcheck
(exception: urllib3 v2.0 only supports OpenSSL 1.1.1+,
currently the 'ssl' module is compiled with OpenSSL
1.0.2n  7 Dec 2017.
See: https://github.com/urllib3/urllib3/issues/2168)

The Gluon issuetracker suggests specifying the build-container.

Link: https://github.com/readthedocs/readthedocs.org/issues/10290#issuecomment-1535120995



Signed-off-by: David Bauer <mail@david-bauer.net>