Err:4 https://download.docker.com/linux/ubuntu focal InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8 code example
Example: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E