This was failing the docker hub builds, since the git-config retains
an absolute path to the worktree location when cloning. Copying it
over from the host system means that this path now points to a
non-existent location, which then interfered with the submodule
initialization.
This fixes it by not using the copy directly, but rather it creates a
clean clone from the copied location, including a submodule init.
Signed-off-by: Christian Decker <@cdecker>
