From command line
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable
$(lsb_release -cs) is "xenial", which is "distribution"
"stable" is components
Similarly, with GUI Softwares & Update, new repository can be added as below
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.
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 -