@olivierlambert Thanks, just stumbled over this a few minutes ago and tried it, but doesn't work. According to https://www.digitalocean.com/community/tutorials/an-introduction-to-cloud-config-scripting#write-files-to-the-disk

This currently only works for RHEL-based distributions.

But this led me to write_files and this works:

write_files:
  - path: /etc/resolv.conf
    content: |
      domain exmple.com
      search example.com
      nameserver 192.168.20.1

this is the full configuration I use:

#cloud-config
hostname: {name}
timezone: Europe/Vienna
users:
  - default
  - name: johndow
    passwd: ****
    sudo: ALL=(ALL) NOPASSWD:ALL
    groups: users, admin, sudo
    shell: /bin/bash
    lock_passwd: true
    ssh_authorized_keys:
      - ssh-ed25519 AAAA****
write_files:
  - path: /etc/resolv.conf
    content: |
      domain example.com
      search example.com
      nameserver 192.168.20.1
package_update: true
packages:
  - sudo
  - htop
  - vim
  - python
  - tmux
package_upgrade: true
runcmd:
  - sudo reboot

the Network configuration looks like this:

  version: 2
  ethernets:
    eth0:
      match:
        name: eth0
      addresses:
        - 192.168.20.6/28
      gateway4: 192.168.20.1
    eth1:
      match:
        name: eth1
      addresses:
        - 10.10.10.6/24

but now it seems I forgot the MTU but this is a different story, thanks for the hint in the right direction.