Issues upgrading NVIDIA container runtime on Ubuntu server 18.04:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://nvidia.github.io/libnvidia-container/ubuntu18.04/amd64 InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://nvidia.github.io/nvidia-container-runtime/ubuntu18.04/amd64 InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://nvidia.github.io/nvidia-docker/ubuntu18.04/amd64 InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: Failed to fetch https://nvidia.github.io/libnvidia-container/ubuntu18.04/amd64/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: Failed to fetch https://nvidia.github.io/nvidia-container-runtime/ubuntu18.04/amd64/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: Failed to fetch https://nvidia.github.io/nvidia-docker/ubuntu18.04/amd64/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6ED91CA3AC1160CD W: Some index files failed to download. They have been ignored, or old ones used instead.
Fixed by:
curl -s -L https://nvidia.github.io/nvidia-container-runtime/gpgkey | sudo apt-key add -