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
.
Sorry! Something went wrong. If you can't figure this out, please copy and paste all this o
utput into the Outline Manager screen, and send it to us, to see if we can help you.
Full log: /tmp/outline_log9Mj
