（注意：如果你此时运行sudo apt-get update，可能会在结尾出现错误：
W：GPG error:
<a target="_blank" href="https://get.docker.io">https://get.docker.io</a>
docker Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D8576A8BA8BD21E9）