Dockerfile to build image from prebuilt tar file

Change-Id: If386255286fea648d112a83a4503701523a78267
diff --git a/.dockerignore b/.dockerignore
index f78caff..10473cf 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -7,6 +7,7 @@
 
 .buckd
 buck-out
+!buck-out/gen/tools/package/onos-package/onos.tar.gz
 bucklets/plugins
 .git
 bin