Remove volume mounted on builds

[humitos]

We shouldn't require this anymore now that we are running the git commands inside the Docker container.

This mount could probably removed completely: https://github.com/readthedocs/common/blob/master/dockerfiles/docker-compose.yml#L110

Allows to avoid VersionLocked exception when working locally with multiple builders (--scale-build 5).

[humitos]

It's not trivial to remove this. It fails with

/bin/sh: 1: cd: can't cd to /tmp/readthdocs_user_builds/sphinx-notfound-page/checkouts/latest

This is because we are creating some directories from the Host and using them from the Container. Example when syncing a repository. All of these directories should be created by calling environment.run(f'mkdir {self.project.doc_path}') (or similar) instead. This way, it will be created locally or inside the container itself.

[humitos]

Note that this will allow us to run all commands as docs users locally instead of using root inside the container's images.

[humitos]

Also note that there are other places that share the mounted volume with the host for specific tasks. For example, here we are reading the git repository with GitPython from the host: https://github.com/readthedocs/readthedocs.org/blob/5.24.0/readthedocs/vcs_support/backends/git.py#L82

The more we can avoid these (or split) the better isolation that we will have.