From https://stackoverflow.com/questions/24418815/how-do-i-instal... , here's an example that you can just paste into your load balancing LaunchConfig and never have to log into an instance at all (just add your own runcmd: section -- and, hey, it's even YAML like everyone loves)

  #cloud-config
  
  apt:
    sources:
      docker.list:
        source: deb [arch=amd64] https://download.docker.com/linux/ubuntu $RELEASE stable
        keyid: 9DC858229FC7DD38854AE2D88D81803C0EBFCD88
  
  packages:
    - docker-ce
    - docker-ce-cli