docker 出错误，应如何修改命令

使用 ubuntu 准备下载dacker，却出现以下的错误，要怎么解决这个问题呢？

Docker installation failed, please visit
https://docs.docker.com/install for instructions.

Last error: # Executing docker install script, commit: 28bc4d09b3938ea30c69407d198ee8ece52c
3e12

• sh -c apt-get update -qq >/dev/null
  W: GPG error: https://download.docker.com/linux/ubuntu bionic InRelease: The following sign
  atures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273
  FCD8
  E: The repository 'https://download.docker.com/linux/ubuntu bionic InRelease' is not signed
  .

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