Docker

Docker is an extensible, open-source engine powered by Linux Containers that automates the deployment of applications as portable, lightweight, and self-sufficient containers. For purposes of this tutorial, we’ll assume you’ve followed the steps outlined in our Setting Up and Securing a Compute Instance, that your system is up to date, and that you’ve logged in to your Linode as root via SSH.

Installation

For the purposes of this guide we will show you how to install Docker on both Ubuntu 12.04 and CentOS 6.4 Docker provides repositories for each of these distributions, which makes installation easy.

Ubuntu 12.04 64bit

Here we will add the Docker-maintained repository for Ubuntu and install the software.

1. Docker is available as a package in Docker’s Ubuntu repositories, but only for 64bit. First, you will need to add the Docker repository key using apt-key:

   sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 36A1D7869245C8950F966E92D8576A8BA88D21E9
   

2. Add the Docker repository to your apt sources:

   echo "deb http://get.docker.io/ubuntu docker main" | sudo tee /etc/apt/sources.list.d/docker.list
   

3. Run the following to apt-get update and install lxc-docker:

   sudo apt-get update
   sudo apt-get install lxc-docker
   

4. To verify that the installation has completed successfully, launch an example Ubuntu container. This command will automatically grab any missing images, run the container, and provide an interactive bash session:

   sudo docker run -i -t ubuntu /bin/bash
   

   The output should resemble:

   # docker run -i -t ubuntu /bin/bash
   Unable to find image 'ubuntu' (tag: latest) locally
   Pulling repository ubuntu
   8dbd9e392a96: Download complete
   b750fe79269d: Download complete
   27cf78414709: Download complete
   root@145dfc4f6dff:/#

5. To exit the container, type exit.

CentOS 6 64bit

Docker is available on the EPEL repository as the docker-io package for 64bit only.

1. To add the EPEL repository, install the latest epel-release package.

2. To install Docker, run the following command:

   sudo yum -y install docker-io
   

3. To start the Docker daemon, invoke service:

   sudo service docker start
   

4. If you would like the Docker daemon to start at boot, issue the following command:

   sudo chkconfig docker on
   

5. To verify that the installation has completed successfully, launch an example Fedora container. This command will automatically grab any missing images, run the container, and provide an interactive bash session:

   sudo docker run -i -t fedora /bin/bash
   

6. To exit the container, type exit.

What Can I Do with Docker?

Docker allows users to package their applications and configurations into lightweight images for deployment as portable containers.

Hello World

To run a Docker container that prints “hello world”, run the following command:

docker run ubuntu /bin/echo hello world

It should return hello world.

This tells Docker to do a number of things:

1. If the image (Ubuntu) doesn’t already exist locally, download it from the Docker Index.
2. Create a new container with the base Ubuntu image with a read-write filesystem and networking interface.
3. Allocate an IP address to the container; set up NAT to forward traffic to and from the container.
4. Run the command /bin/echo hello world, and print the output.
5. Having completed the main process, the container will exit.

Writing a Dockerfile for Nginx

When building an image, Docker follows the instruction set in a file named Dockerfile. Note that the file needs to be named Dockerfile and all any files or folders required by the Dockerfile need to be under the same directory as the Dockerfile itself.

Running a program like echo in a Docker container is pretty simple. However, for programs that act as servers, such as Nginx, you will need to ensure that the program is configured not to self-daemonize.

This is an example Dockerfile for Nginx:

FROM        ubuntu:12.04
MAINTAINER  Jon Chen "fly@burrito.sh"

RUN         echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN         apt-get update
RUN         apt-get install -y nginx

RUN         echo "\ndaemon off;" >> /etc/nginx/nginx.conf
VOLUME      /etc/nginx/sites-enabled
VOLUME      /var/log/nginx

EXPOSE      80
CMD         ["nginx"]

The Dockerfile syntax looks like this:

# Comment
INSTRUCTION arguments

Docker runs through the Dockerfile instructions from top to bottom in order. The first instruction must be FROM, which specifies the base image from which you wish to build your new image:

FROM ubuntu:12.04

This sets the official Ubuntu 12.04 image as the base image. You will also wish to use the MAINTAINER instruction to define the author of the image:

MAINTAINER Jon Chen "fly@burrito.sh"

Next, the RUN instruction will execute commands on the image, and commit the results. Each commit is saved and used for the next instruction. For example, this RUN line replaces the contents of /etc/apt/sources.list in the image with deb http://archive.ubuntu.com/ubuntu precise main universe:

RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list

The important thing to remember for programs such as Nginx is to ensure that the program doesn’t run as a daemon. Nginx’s default behavior as a daemon is to fork off worker processes, then exit the master process. As Docker only watches the PID for the original process, the container will halt prematurely instead of running persistently. To disable daemonization, add the daemon off configuration directive to /etc/nginx/nginx.conf:

RUN echo "\ndaemon off;" >> /etc/nginx/nginx.conf

To expose a port inside the container to the outside of the container, use the EXPOSE instruction:

EXPOSE 80

The CMD instruction defines the default command to run when the container starts. In our example, we want to run Nginx:

CMD ["nginx"]

By default, Docker containers do not have persistent storage. In order to share data between containers, use the VOLUME feature:

VOLUME /etc/nginx/sites-enabled
VOLUME /var/log/nginx

In order to mount a directory from the host onto the container, you will need to specify the host directory, corresponding container directory, and directory permissions in the command line when you run the container:

-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.

To build this image, run the following command in the same directory as the Dockerfile. You can specify a repository and tag for your image as well with -t repo/tag:

docker build -t bsdlp/nginx .

Run the following command to add /etc/nginx/sites-enabled and /var/log/nginx as volumes from the host to the container, start the container as a daemon, and expose port 80 from the container as port 80 on the host:

docker run -d -p 80:80 -v /etc/nginx/sites-enabled:/etc/nginx/sites-enabled -v /var/log/nginx:/var/log/nginx bsdlp/nginx

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Docker’s Getting Started guide
• Docker on GitHub
• Official Docker Image Index