Docker Cookbook

172 | Chapter 6: Optimized Operating System Distributions for Docker

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6.2 Starting a Container on CoreOS via cloud-init
Problem
Knowing how to start a CoreOS instance via Vagrant, you would like to use 
cloud-
init
 to start a container at boot time.
Solution
You know how to start a CoreOS instance via Vagrant (see 
Recipe 6.1
). You now need
to add a 
systemd
unit within the 
user-data
file. CoreOS will automatically launch this
unit during the boot process.
Create a new 
user-data
file that contains only the following:
#cloud-config
coreos:
units:
- name: es.service
command: start
content: |
[Unit]
After=docker.service
Requires=docker.service
Description=starts Elastic Search container
[Service]
TimeoutStartSec=0
ExecStartPre=/usr/bin/docker pull dockerfile/elasticsearch
ExecStart=/usr/bin/docker run -d -p 9200:9200 -p 9300:9300 \
dockerfile/elasticsearch
If you still have a CoreOS instance running from 
Recipe 6.1
, destroy it with 
vagrant
destroy
and bring up a new one with 
vagrant up
.
The 
docker.service
unit starts automatically in CoreOS, so there
is no need to specify it in the 
user-data
file.
The virtual machine will boot quickly and start the 
es.service
defined in the cloud
config file. Docker will start by pulling the 
dockerfile/elasticsearch
image. This could
take some time, so be patient and monitor the download via 
docker images
. Once
the image is downloaded, the container will get started (see the 
ExecStart
command
in the 
user-data
file):