libnetwork: check DNS loopback with user DNS opts

DNS servers in the loopback address range should always be resolved in
the host network namespace when the servers are configured by reading
from the host's /etc/resolv.conf. The daemon mistakenly conflated the
presence of DNS options (docker run --dns-opt) with user-supplied DNS
servers, treating the list of servers loaded from the host as a user-
supplied list and attempting to resolve in the container's network
namespace. Correct this oversight so that loopback DNS servers are only
resolved in the container's network namespace when the user provides the
DNS server list, irrespective of other DNS configuration.

Signed-off-by: Cory Snider <csnider@mirantis.com>

libnetwork/sandbox_dns_unix.go

@@ -258,7 +258,7 @@ func (sb *Sandbox) setupDNS() error {
 		// After building the resolv.conf from the user config save the
 		// external resolvers in the sandbox. Note that --dns 127.0.0.x
 		// config refers to the loopback in the container namespace
-		sb.setExternalResolvers(newRC.Content, resolvconf.IPv4, false)
+		sb.setExternalResolvers(newRC.Content, resolvconf.IPv4, len(sb.config.dnsList) == 0)
 	} else {
 		// If the host resolv.conf file has 127.0.0.x container should
 		// use the host resolver for queries. This is supported by the