Remove duplication of Dns in config merging.

Fixes #4714
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

runconfig/config_test.go

@@ -231,7 +231,7 @@ func TestMerge(t *testing.T) {
 	volumesUser := make(map[string]struct{})
 	volumesUser["/test3"] = struct{}{}
 	configUser := &Config{
-		Dns:       []string{"3.3.3.3"},
+		Dns:       []string{"2.2.2.2", "3.3.3.3"},
 		PortSpecs: []string{"3333:2222", "3333:3333"},
 		Env:       []string{"VAR2=3", "VAR3=3"},
 		Volumes:   volumesUser,

runconfig/merge.go

@@ -97,8 +97,15 @@ func Merge(userConf, imageConf *Config) error {
 	if userConf.Dns == nil || len(userConf.Dns) == 0 {
 		userConf.Dns = imageConf.Dns
 	} else {
-		//duplicates aren't an issue here
-		userConf.Dns = append(userConf.Dns, imageConf.Dns...)
+		dnsSet := make(map[string]struct{}, len(userConf.Dns))
+		for _, dns := range userConf.Dns {
+			dnsSet[dns] = struct{}{}
+		}
+		for _, dns := range imageConf.Dns {
+			if _, exists := dnsSet[dns]; !exists {
+				userConf.Dns = append(userConf.Dns, dns)
+			}
+		}
 	}
 	if userConf.Entrypoint == nil || len(userConf.Entrypoint) == 0 {
 		userConf.Entrypoint = imageConf.Entrypoint