How to use cloud-init with a debian-based image on Google Cloud?

To complement answer from @norbjd, posting this for completeness:

• in order to install kubectl package that I planned initially, I had to write this cloud-init config:

  #cloud-config
  
  bootcmd:
   - curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add
  
  apt:
    sources:
      kubernetes.list:
        source: "deb http://apt.kubernetes.io/ kubernetes-xenial main"
  
  packages:
   - kubectl
  

• if you need to know which images contain which packages, these resources may be helpful:

  • Ubuntu:

    • Ubuntu Cloud Images (RELEASED)

    • Actual contents (example):

      • https://cloud-images.ubuntu.com/releases/releases/18.04/release-20191003/ubuntu-18.04-server-cloudimg-amd64.manifest

  • Debian:

    • List of differences between standard and cloud systems

    • Actual contents:

      • https://salsa.debian.org/cloud-team/debian-cloud-images/blob/master/config_space/package_config/EXTRAS
      • https://salsa.debian.org/cloud-team/debian-cloud-images/blob/master/config_space/package_config/CLOUD

cloud-init is installed on the latest (at the moment of writing) Ubuntu 18.04 LTS (ubuntu-1804-bionic-v20191002) image :

<my_user>@instance-1:~$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.3 LTS"

<my_user>@instance-1:~$ which cloud-init
/usr/bin/cloud-init

You should replace debian-cloud/debian-9 with ubuntu-os-cloud/ubuntu-1804-bionic-v20191002.