Multiprocess Docker image creation for GHCR fails

[jjeffers]

I've noticed that the CI action for the GHCR image build fails with a repository key error.

#10 7.855 W: GPG error: http://repo.mysql.com/apt/ubuntu bionic InRelease: 
   The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7B3B788A8D3785C
#10 7.855 E: The repository 'http://repo.mysql.com/apt/ubuntu bionic InRelease' is not signed.