Checkout submodules in Docker Hub hook

hooks/pre_build

@@ -1,4 +1,6 @@
 #!/bin/bash
+git submodule update --init --recursive
+
 # Register qemu-*-static for all supported processors except the 
 # current one, but also remove all registered binfmt_misc before
 docker run --rm --privileged multiarch/qemu-user-static:register --reset