UseDefaultTag for skip build if not default branch.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

cmd/drone-docker/main.go

@@ -246,11 +246,19 @@ func run(c *cli.Context) error {
 	}
 
 	if c.Bool("tags.auto") {
-		plugin.Build.Tags = docker.DefaultTagSuffix(
+		if docker.UseDefaultTag( // return true if not default branch, or not tag
 			c.String("commit.ref"),
 			c.String("repo.branch"),
-			c.String("tags.suffix"),
-		)
+		) {
+			plugin.Build.Tags = docker.DefaultTagSuffix(
+				c.String("commit.ref"),
+				c.String("tags.suffix"),
+			)
+		} else {
+			logrus.Printf("skipping automated docker build for %s", c.String("commit.ref"))
+
+			return nil
+		}
 	}
 
 	return plugin.Exec()