Execute checkout step respecting docker setting

[stsewd]

Closes #5673

There is a couple of things with this:

• Git commands using gitpython will use the git binary from the host
• Git commands using git directly, will use the git binary from the
  container.

This may bring some surprises, since the git versions will be different.
But I'm not really worried about it, it should work.

Another thing is: To make a clone we will spin a build image, which are
big, probably we could add an image that only has git in it.

This should be merged after we have integrated this with the ssh-agent
in .com

[humitos]

I just tested this and it worked.

With this setup, I think we can spin up just one container and do everything there without the need to have setup_env and build_env, right?

[stsewd]

With this setup, I think we can spin up just one container and do everything there without the need to have setup_env and build_env, right?

Nope, we still require two env. Because to have only one env, we'll need to know the image to be used, that is known after parsing the config file.

[humitos]

This may bring some surprises, since the git versions will be different.
But I'm not really worried about it, it should work.

I agree. I don't think this will be a real problem unless the git from the container updates to a version that is not backward compatible. Although, we can easily pin the version that we want to use.

To make a clone we will spin a build image, which are big, probably we could add an image that only has git in it.

This is probably a good thing to do. Besides, it's really simple. I just did a test:

# Dockerfile
FROM ubuntu:18.04

RUN apt-get update
RUN apt-get install -y git

This docker image should have all the VCS that we support installed

and then create a class for the setup step:

class DockerSetupEnvironment(DockerBuildEnvironment)

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.container_image = 'readthedocs/git:latest'

I'm 👍 with this solution.

[ericholscher]

This feels like a reasonable solution to me. It's a bit hacky, but way less complexity than the other approaches we've tried.

[stsewd]

This isn't blocked anymore, ssh is working with docker now.

[davidfischer]

I tested this in development without issue! I'm excited to finally have this. While git doesn't frequently have security issues, I'm glad to at least have some mitigation in place for that.

[davidfischer]

Minor nitpick: the comment above this line (which I can't comment on directly) should be updated.