Multi-arch build

Currently, for amd64 (the default) and arm.
Run `dapper make ARCH=arm rootfs` to build rootfs.tar.gz for arm.

init/bootstrap.go

@@ -1,7 +1,6 @@
 package init
 
 import (
-	"os"
 	"syscall"
 
 	"fmt"
@@ -61,7 +60,7 @@ func stopDocker(c chan interface{}) error {
 	c <- struct{}{}
 	<-c
 
-	return os.RemoveAll(config.DOCKER_SYSTEM_HOME)
+	return nil
 }
 
 func bootstrap(cfg *config.CloudConfig) error {