builder/docker: error if DOCKER_HOST is set

CHANGELOG.md

@@ -55,6 +55,7 @@ BUG FIXES:
   * builder/amazon/all: Fix race condition on setting tags. [GH-1367]
   * builder/amazon/all: More desctriptive error messages if Amazon only
       sends an error code. [GH-1189]
+  * builder/docker: Error if `DOCKER_HOST` is set.
   * builder/docker: Remove the container during cleanup. [GH-1206]
   * builder/docker: Fix case where not all output would show up from
       provisioners.

builder/docker/driver_docker.go

@@ -220,5 +220,10 @@ func (d *DockerDriver) Verify() error {
 		return err
 	}
 
+	if v := os.Getenv("DOCKER_HOST"); v != "" {
+		return fmt.Errorf(
+			"DOCKER_HOST cannot be set with the Packer Docker builder.")
+	}
+
 	return nil
 }