Add support for a RC1 image with Python 3.8

[humitos]

This PR add support to start testing a RC1 Docker image with support for 3.8.

Related #6324 readthedocs/readthedocs-docker-images#111

[humitos]

I just realized that it's better if we only upload the image to build servers and change container_image from the admin instead of allowing a "temporal" image in our config file. I'm closing this.

[humitos]

Hrm... I'm not sure what's the pattern we can follow here to support a testing image where we can select the Python interpreter. I think we don't have one for this already.

I'm thinking that it could be a feature flag to force python3.8 as the interpreter for a specific project. So, we will need to set this flag + project.container_image = 'readthedocs/build:6.0rc1'

[stsewd]

We shouldn't change the python interpreter to people that have a specific version. Putting the image on the to 6.0rc1 should be enough, so people using python: 3 get the highest version, 3.8 for this image.

But I think we need to change the code to override the one from the config file, currently the docker image is being overridden in a later step.

[stsewd]

But I think we need to change the code to override the one from the config file, currently the docker image is being overridden in a later step.

Nope, I got confused with the USE_TESTING_BUILD_IMAGE (wich actually is allways overriden by the config file, the config always have a build.image attribute).

readthedocs.org/readthedocs/doc_builder/environments.py

Lines 810 to 811 in 417e221

	if self.config and self.config.build.image:
	self.container_image = self.config.build.image