This commit is to set dns search to null in the harbor containers,
that means the dns search domains of docker host doesn't impact
the network IO in the containers.

If do not set this, Harbor notary-server and notary-signer are resolving
the "mysql" alias to the resolv.conf search path instead of to "mysql."
for the notary-db bridge IP, see #6031.

According to docker official document, 'Use --dns-search=.' if you don't
wish to set the search domain.

https://docs.docker.com/v17.09/engine/userguide/networking/default_network/configure-dns/

Signed-off-by: wang yan <wangyan@vmware.com>