docker: allow running from srcdir != builddir build

The new script uses "git submodule", which is picky about being invoked
from the top of the git checkout.  Invoke the script from $(SRC_PATH)
to avoid git's wrath.

Fixes: b7f404201e45e99da23b9764dec27ce5f965cdcd
Cc: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Message-Id: <1508331989-142364-1-git-send-email-pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
