de-mangle windows filepath to something docker can handle

builder/docker/driver_docker.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/exec"
 	"regexp"
+	"runtime"
 	"strings"
 	"sync"
 
@@ -227,6 +228,11 @@ func (d *DockerDriver) StartContainer(config *ContainerConfig) (string, error) {
 		args = append(args, "--privileged")
 	}
 	for host, guest := range config.Volumes {
+		if runtime.GOOS == "windows" {
+			// docker-toolbox can't handle the normal C:\filepath format in CLI
+			host = strings.Replace(host, "\\", "/", -1)
+			host = strings.Replace(host, "C:/", "/c/", 1)
+		}
 		args = append(args, "-v", fmt.Sprintf("%s:%s", host, guest))
 	}
 	for _, v := range config.RunCommand {