Docker inside CT

Since OpenVZ kernel 042stab105.4 it is possible to run Docker inside containers. This article describes how.

Prerequisites

• Kernel 042stab105.4 or later version
• Kernel module veth module is loaded on host

Container tuning

• 1. Create Fedora 20 container:

vzctl create $veid --ostemplate fedora-20-x86_64

• 2. Turn on bridge feature to allow docker creating bridged network:

vzctl set $veid --features bridge:on --save

• 3. Setup Container veth-based network:

vzctl set $veid --netif_add eth0 --save

• 4. Allow all iptables modules being used in containers:

vzctl set $veid --netfilter full --save

• 5. Configure custom cgroups in systemd:

vzctl mount $veid
echo "JoinControllers=cpu,cpuacct,cpuset,freezer,devices" >> /vz/root/$veid/etc/systemd/system.conf 

• 6. Start the container:

vzctl start $veid

Prepare Docker in container

These steps are to be performed inside the container.

• 1. Install Docker:

yum -y install docker-io

• 2. Start docker daemon

 docker -d -s vfs

Example usage

Wordpress

Use Docker to start Wordpress (official, standard way).

• 1. Start mysql docker:

docker run --name test-mysql -e MYSQL_ROOT_PASSWORD=123 -d mysql

• 2. Start wordpress:

docker run --name test-wordpress --link test-mysql:mysql -p 8080:80 -d wordpress

• 3. Access wordpress server by container IP and port 8080:

  http://container_ip:8080

Limitations

• This feature is currently in beta
• Only "vfs" Docker graph driver is currently supported