docker build fails at apt-get update because public key is not available: NO_PUBKEY

[FrancescoCasalegno]

docker build morphoclass started failing today at the step where we apt-get update.
This seems to be caused by this known issue: NVIDIA/nvidia-container-toolkit#257

The error message is the following:

Step 8/24 : RUN apt-get update && DEBIAN_FRONTEND="noninteractive" apt-get install -y --no-install-recommends     build-essential make curl git htop less man tzdata vim wget
 ---> Running in f3ceb0879622
Get:1 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64  InRelease [1581 B]
Err:1 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64  InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A4B469963BF863CC
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:3 http://archive.ubuntu.com/ubuntu bionic InRelease [242 kB]
Ign:4 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64  InRelease
Get:5 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64  Release [564 B]
Get:6 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64  Release.gpg [833 B]
Get:7 https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64  Packages [73.8 kB]
Get:8 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [2762 kB]
Get:9 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:10 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:11 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages [1344 kB]
Get:12 http://archive.ubuntu.com/ubuntu bionic/restricted amd64 Packages [13.5 kB]
Get:13 http://security.ubuntu.com/ubuntu bionic-security/restricted amd64 Packages [932 kB]
Get:14 http://archive.ubuntu.com/ubuntu bionic/multiverse amd64 Packages [186 kB]
Get:15 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [1503 kB]
Get:16 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages [11.3 MB]
Get:17 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 Packages [21.1 kB]
Get:18 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [3197 kB]
Get:19 http://archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 Packages [29.8 kB]
Get:20 http://archive.ubuntu.com/ubuntu bionic-updates/restricted amd64 Packages [966 kB]
Get:21 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages [2277 kB]
Get:22 http://archive.ubuntu.com/ubuntu bionic-backports/main amd64 Packages [12.2 kB]
Get:23 http://archive.ubuntu.com/ubuntu bionic-backports/universe amd64 Packages [12.9 kB]
Reading package lists...
W: GPG error: https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64  InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A4B469963BF863CC
E: The repository 'https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64  InRelease' is not signed.