Installing docker-compose using coreos ignition

Question

I know how to install using cloud-config. But since this is a first-boot only task, I would instead like to install it using ignition.

Here is my attempted configuration (that doesn't work):

"systemd": {
  "units": [{
    "name": "install-docker-compose.service",
    "contents": "[Unit]\nDescription=Install docker-compose\nConditionPathExists=!/opt/bin/docker-compose\n[Service]\nType=oneshot\nRemainAfterExit=yes\nExecStart=/usr/bin/mkdir -p /opt/bin/\nExecStart=/usr/bin/curl --create-dirs -o /opt/bin/docker-compose -sL \"https://github.com/docker/compose/releases/download/1.9.0/docker-compose-linux-x86_64\"\nExecStart=/usr/bin/chmod +x /opt/bin/docker-compose"
  }]
}