Build: spin only one Docker container for whole build process

[humitos]

Currently, we are spinning up 2 Docker containers:

1. use readthedocs/build:ubuntu-20.04 to run setup VCS process
2. read .readthedocs.yaml's build.image or build.os key to decide what Docker container image spin depending on user's configuration to build the docs

It would be good if we can reduce the complexity and resource usage to spin up just one image for the whole process. However, we need to find a way to get the config file without cloning the repository so we can spin up initially the image the user has selected in the config file for the build process.

I thought we could use GitHub's API to retrieve the content of the .readthedocs.yaml, but we would need a similar implementation for other VCS providers as well. I'm not sure what's the best solution here, but it would be good to brainstorm some ideas and check how other CIs do this.

[stsewd]

We also need to do 4 calls to check if file exists (to check all valid names of the config file).

but we would need a similar implementation for other VCS providers as well

And for people that doesn't use a supported provider (github/gitlab/bitbucket).

But we can try a mix, use one method for the supported providers, and use the old method for the others.

[humitos]

Another problem related to using two different Docker images in the build process: #8995

[humitos]

We are going to use the same Docker image now #10555. We can probably keep this pattern from now on each time a new Ubuntu LTS is released.