Use keyserver.ubuntu.com as a keyserver for CRAN

$ docker run -it --rm openjdk:11 /bin/bash
root@3e89a8d05378:/# echo "deb http://cloud.r-project.org/bin/linux/debian buster-cran35/" >> /etc/apt/sources.list
root@3e89a8d05378:/# (apt-key adv --keyserver keys.gnupg.net --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF' || apt-key adv --keyserver keys.openpgp.org --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF')
Executing: /tmp/apt-key-gpghome.8lNIiUuhoE/gpg.1.sh --keyserver keys.gnupg.net --recv-key E19F5F87128899B192B1A2C2AD5F960A256A04AF
gpg: keyserver receive failed: No name
Executing: /tmp/apt-key-gpghome.stxb8XUlx8/gpg.1.sh --keyserver keys.openpgp.org --recv-key E19F5F87128899B192B1A2C2AD5F960A256A04AF
gpg: key AD5F960A256A04AF: new key but contains no user ID - skipped
gpg: Total number processed: 1
gpg:           w/o user IDs: 1
root@3e89a8d05378:/# apt-get update
...
Err:3 http://cloud.r-project.org/bin/linux/debian buster-cran35/ InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FCAE2A0E115C3D8A
...
W: GPG error: http://cloud.r-project.org/bin/linux/debian buster-cran35/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FCAE2A0E115C3D8A
E: The repository 'http://cloud.r-project.org/bin/linux/debian buster-cran35/ InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

$ docker run -it --rm openjdk:11 /bin/bash
root@c9b183e45ffe:/# echo "deb http://cloud.r-project.org/bin/linux/debian buster-cran35/" >> /etc/apt/sources.list
root@c9b183e45ffe:/# apt-key adv --keyserver keyserver.ubuntu.com --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF'
Executing: /tmp/apt-key-gpghome.P6cxYkOge7/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-key E19F5F87128899B192B1A2C2AD5F960A256A04AF
gpg: key AD5F960A256A04AF: public key "Johannes Ranke (Wissenschaftlicher Berater) <johannes.ranke@jrwb.de>" imported
gpg: Total number processed: 1
gpg:               imported: 1
root@c9b183e45ffe:/# apt-get update
Get:1 http://deb.debian.org/debian buster InRelease [122 kB]
Get:2 http://security.debian.org/debian-security buster/updates InRelease [65.4 kB]
Get:3 http://cloud.r-project.org/bin/linux/debian buster-cran35/ InRelease [4375 B]
Get:4 http://deb.debian.org/debian buster-updates InRelease [51.9 kB]
Get:5 http://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages [53.3 kB]
Get:6 http://security.debian.org/debian-security buster/updates/main arm64 Packages [287 kB]
Get:7 http://deb.debian.org/debian buster/main arm64 Packages [7735 kB]
Get:8 http://deb.debian.org/debian buster-updates/main arm64 Packages [14.5 kB]
Fetched 8334 kB in 2s (4537 kB/s)
Reading package lists... Done