Comment 3 for bug 1905932

I improved the first workaround

  early-commands:
    # Get hostname from DNS
    - host $(/usr/sbin/ip a show dev $(/usr/sbin/ip route show default | awk 'sub(/.*dev /,"",$0) {print $1; exit;}') | awk '/inet / {sub(/\/.*/,"",$2); print $2}') | awk 'sub(/\..*/,"",$NF) { print $NF }' > /etc/hostname
    - hostname $(cat /etc/hostname)

Another one is to use json output of ip binary

  early-commands:
    # Get hostname from DNS
    - apt update && apt install -y jq
    - host $(/usr/sbin/ip -4 -j a show $(/usr/sbin/ip -j route show match default | jq -re '.[0].dev') | jq -re '.[0].addr_info[0].local') | awk 'sub(/\..*/,"",$NF) { print $NF }' > /etc/hostname
    - hostname $(cat /etc/hostname)