Escapes cp alias

Fix for #1656

builder/docker/communicator.go

@@ -75,7 +75,7 @@ func (c *Communicator) Upload(dst string, src io.Reader, fi *os.FileInfo) error
 	// Copy the file into place by copying the temporary file we put
 	// into the shared folder into the proper location in the container
 	cmd := &packer.RemoteCmd{
-		Command: fmt.Sprintf("cp %s/%s %s", c.ContainerDir,
+		Command: fmt.Sprintf("\cp %s/%s %s", c.ContainerDir,
 			filepath.Base(tempfile.Name()), dst),
 	}