fix docker build

add build folders to docker ignore to fix problems when building the
image when the daemon has already been built locally

Change-Id: Ie24039ee5fbbf8944a008b6d0deac61a8400d00b

.dockerignore

@@ -6,4 +6,6 @@ contrib/arm*
 contrib/aarch64*
 contrib/i686*
 
+**/build/
+
 .git/