user@host $ docker run --pull=always -t -it --rm debian:bookworm bash
bookworm: Pulling from library/debian
8cd46d290033: Pull complete
Digest: sha256:b8084b1a576c5504a031936e1132574f4ce1d6cc7130bbcc25a28f074539ae6b
Status: Downloaded newer image for debian:bookworm

root@5b3bae13b719:/# apt update
[ snip ]
root@5b3bae13b719:/# apt -y install apt-transport-https wget
[ snip ]
root@5b3bae13b719:/# wget -O icinga-archive-keyring.deb "https://packages.icinga.com/icinga-archive-keyring_latest+debian$(
 . /etc/os-release; echo "$VERSION_ID"
).deb"
[ snip ]
root@5b3bae13b719:/# apt install ./icinga-archive-keyring.deb
[ snip ]
root@5b3bae13b719:/# DIST=$(awk -F"[)(]+" '/VERSION=/ {print $2}' /etc/os-release); \
 echo "deb https://packages.icinga.com/debian icinga-${DIST} main" > \
 /etc/apt/sources.list.d/${DIST}-icinga.list
 echo "deb-src https://packages.icinga.com/debian icinga-${DIST} main" >> \
 /etc/apt/sources.list.d/${DIST}-icinga.list

root@5b3bae13b719:/# apt update
Hit:1 http://deb.debian.org/debian bookworm InRelease
Hit:2 http://deb.debian.org/debian bookworm-updates InRelease
Hit:3 http://deb.debian.org/debian-security bookworm-security InRelease
Get:4 https://packages.icinga.com/debian icinga-bookworm InRelease [11.7 kB]
Err:4 https://packages.icinga.com/debian icinga-bookworm InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C6E319C334410682
Reading package lists... Done
W: GPG error: https://packages.icinga.com/debian icinga-bookworm InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C6E319C334410682
E: The repository 'https://packages.icinga.com/debian icinga-bookworm 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.

$ gpg --search-key C6E319C334410682
gpg: data source: https://keys.openpgp.org:443
(1)       1024 bit DSA key C6E319C334410682, created: 2013-11-16