Fix GitLab Runner public key error (The following signatures couldn't be verified...)

Sometimes `apt-get update` fails with a NO_PUBKEY error when the repository public key is missing. Example:

Update attempt output

# apt-get update
...
Get:8 https://packages.gitlab.com/runner/gitlab-runner/debian stretch InRelease [23.4 kB]
Err:8 https://packages.gitlab.com/runner/gitlab-runner/debian stretch InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3F01618A51312F3F
...
Fetched 23.4 kB in 4s (4,794 B/s)
Reading package lists... Done
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/runner/gitlab-runner/debian stretch 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/runner/gitlab-runner/debian/dists/stretch/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.

Import missing key

The missing key can be retrieved from keyserver.ubuntu.com using `apt-key adv --keyserver keyserver.ubuntu.com --recv-keys`.

# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3F01618A51312F3F
Executing: /tmp/apt-key-gpghome.KMLfZ7U2tz/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 3F01618A51312F3F
gpg: key 3F01618A51312F3F: public key "GitLab B.V. (package repository signing key) <packages@gitlab.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1

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