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Ubuntu APT demystify
  1. Add repository:

    1. From command line
      sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable

      1. $(lsb_release -cs) is "xenial", which is "distribution"

      2. "stable" is components

    2. Similarly, with GUI Softwares & Update, new repository can be added as below

  2. Certificate

    1. When apt-get update, get error message:
      W: GPG error: https://download.docker.com/linux/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8
      W: The repository 'https://download.docker.com/linux/ubuntu xenial InRelease' is not signed.
      N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

    2. Fix it by download the PGP key from repository and add it with apt-key add
      curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -