Docker: Docker のインストール中に Ubuntu で公開鍵エラーを解決する方法 質問する

Ubuntu サーバーに docker と kubernetes をインストールするために以下のコマンドを実行すると、以下のエラー メッセージが表示されます。

root@master:/home/ubuntu# add-apt-repository \
>   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
>   $(lsb_release -cs) \
>   stable"
Hit:1 http://in.archive.ubuntu.com/ubuntu bionic InRelease
Get:2 https://download.docker.com/linux/ubuntu bionic InRelease [64.4 kB]
Hit:3 http://in.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:4 http://in.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:5 http://in.archive.ubuntu.com/ubuntu bionic-security InRelease
**Err:2 https://download.docker.com/linux/ubuntu bionic InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8**
Reading package lists... Done
W: GPG error: https://download.docker.com/linux/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8
**E: The repository 'https://download.docker.com/linux/ubuntu bionic InRelease' is not signed.**
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root@master:/home/ubuntu#

root@master:/# sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7EA0A9C3F273FCD8
Executing: /tmp/apt-key-gpghome.rDOuMCVLF2/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 7EA0A9C3F273FCD8
gpg: keyserver receive failed: No keyserver available