訊息類似這樣：
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 were invalid: EXPKEYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
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 were in
valid: EXPKEYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
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 were invalid: EXPK
EYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
W: Failed to fetch https://nvidia.github.io/libnvidia-container/ubuntu18.04/amd64/InRelease The following signatures were invalid: EXPKEYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
W: Failed to fetch https://nvidia.github.io/nvidia-container-runtime/ubuntu18.04/amd64/InRelease The following signatures were invalid: EXPKEYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
W: Failed to fetch https://nvidia.github.io/nvidia-docker/ubuntu18.04/amd64/InRelease The following signatures were invalid:
EXPKEYSIG 6ED91CA3AC1160CD NVIDIA CORPORATION (Open Source Projects) <cudatools@nvidia.com>
W: Some index files failed to download. They have been ignored, or old ones used instead.解決方法有很多，不過我從上面的訊息發現全部都跟 NVIDIA docker 有關係，於是去找了 NVIDIA docker 的 GPG key 來新增到 apt-key，作法如下：
curl -s -L https://nvidia.github.io/nvidia-docker/gpgkey | \
sudo apt-key add -然後再跑一次原本 apt-add-repository...的指令就搞定了。
