root@gitlab:~# apt update
Hit:1 http://mirrors.digitalocean.com/ubuntu xenial InRelease
Hit:2 http://mirrors.digitalocean.com/ubuntu xenial-updates InRelease
Hit:3 http://mirrors.digitalocean.com/ubuntu xenial-backports InRelease
Get:4 http://security.ubuntu.com/ubuntu xenial-security InRelease [109 kB]
Get:5 https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu xenial InRelease [23.3 kB]
Err:5 https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu xenial InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
Fetched 133 kB in 0s (143 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
117 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: Failed to fetch https://packages.gitlab.com/gitlab/gitlab-ce/ubuntu/dists/xenial/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
W: Some index files failed to download. They have been ignored, or old ones used instead.