Why?I need to install linux-headers in a container.
You need to add e.g. the non-subscription repository to your container and install the headers then.but the typical apt install linux-headers faisl as well as install pve-headers
In this case you need to use a VM and not a a container.A software install process asks me to install
What exactly?A software install process asks me to install
You can also copy the headers from the host to the guest. But keep in mind that you will not be able to load the module from the guest. So if your software requires the module in order to run, it won't be able to run in your container either, so you should, as @tom already suggested, move to a VM.Still the same problem
my sources-list looks like this ...
deb http://ftp.debian.org/debian buster main contrib
deb http://ftp.debian.org/debian buster-updates main contrib
deb http://security.debian.org buster/updates main contrib
deb http://download.proxmox.com/debian bullseye pve-no-subscription
deb http://security.debian.org bullseye/updates main contrib