Default cache volume when registering windows docker executor

Description

When a user registers a new docker executor on windows, we create a default value of /cache. The same thing needs to done for windows but using a windows path.

Proposal

if runtime.GOOS == "windows" {
	s.Docker.Volumes = append(s.Docker.Volumes, "c:\\gitlab-runner\\cache")
} else {
	s.Docker.Volumes = append(s.Docker.Volumes, "/cache")
}

As suggested in !706 (diffs)