Supports git clone via ssh

Description

• problem: git clone (via. http(s)) is very slow than via ssh
• benefits: speed up time to build

Proposal

I have very heavy git repository. (about 3.6GB)

So git cloning via http(s) is very slow. (about 5 minutes with GIT_DEPTH: "1" 😭)

If use git cloning via ssh, end in 1 minute.

Now, clone_url doesn't seem to support cloning via ssh.

• https://gitlab.com/gitlab-org/gitlab-runner/blob/v11.7.0/docs/configuration/advanced-configuration.md#how-clone_url-works
• https://gitlab.com/gitlab-org/gitlab-runner/blob/v11.7.0/common/build.go#L585

Cloned git repositories are cached in runner local disk. But runner(machine) is deleted frequently when using Autoscaling GitLab Runner on AWS

If clone_url supports ssh, my slow build problem will be solved.

Links to related issues and merge requests / references

none