message: Add a link to Dockerfile.
diff: @@ -6,6 +6,8 @@ Docker containers are available to build Mozc binaries for Android JNI library a
  ## System Requirements
  Currently, only Ubuntu 20.04 is tested to host the Docker container to build Mozc.
  +* [Dockerfile](https://github.com/google/mozc/blob/master/docker/ubuntu20.04/Dockerfile) for Ubuntu 20.04
  +
  ## Set up Ubuntu 20.04 Docker container
  ```
  