How to scale Docker containers in production

Deis automates scaling of Docker containers (among other things).

Deis (pronounced DAY-iss) is an open source PaaS that makes it easy to deploy and manage applications on your own servers. Deis builds upon Docker and CoreOS to provide a lightweight PaaS with a Heroku-inspired workflow.

Here is the developer workflow:

deis create myapp                      # create a new deis app called "myapp"
git push deis master                   # built with a buildpack or dockerfile
deis scale web=16 worker=4             # scale up docker containers

Deis automatically deploys your Docker containers across a CoreOS cluster and configures the Nginx routers to route requests to healthy Docker containers. If a host dies, containers are automatically restarted on another host in seconds. Just browse to the proxy URL or use deis open to hit your app.

Some other useful commands:

deis config:set DATABASE_URL=          # attach to a database w/ an envvar
deis run make test                     # run ephemeral containers for one-off tasks
deis logs                              # get aggregated logs for troubleshooting
deis rollback v23                      # rollback to a prior release

To see this in action, check out the terminal video at http://deis.io/overview/. You can also learn about Deis concepts or jump right into deploying your own private PaaS.

Update: 2019-03-11

First of all thanks for those who have upvoted this answer over the years.

Please be aware that this question was asked in August 2013, when Docker was still a very new technology. Since then: Kubernetes was launched on June 2014, Docker swarm was integrated into the Docker engine in Feb 2015, Amazon launched it's container solution, ECS, in April 2015 and Google launched GKE in August 2015. It's fair to say the production container landscape has changed substantially.

The short answer is that you'd have to write your own logic to do this.

I would expect this kind of feature to emerge from the following projects, built on top of docker, and designed to support applications in production:

flynn
deis
coreos
Mesos

Update 1

Another related project I recently discovered:

maestro

Update 2

The latest release Openstack contains support for managing Docker containers:

Docker Openstack
Paas zone within OpenStack

Update 3

System for managing Docker instances

Shipyard

And a presentation on how to use tools like Packer, Docker and Serf to deliver an immutable server infrastructure pattern

FutureOps with Immutable Infrastructure
Slides

Update 4

A neat article on how to wire together docker containers using serf:

Decentralizing Docker: How to use serf with Docker

Update 5

Run Docker on Mesos using the Marathon framework

Mesosphere Docker Developer Tutorial

Update 6

Run Docker on Tsuru as it supports docker-cluster and segregated scheduler deploy

http://blog.tsuru.io/2014/04/04/running-tsuru-in-production-scaling-and-segregating-docker-containers/

Update 7

Docker-based environments orchestration

maestro-ng

Update 8

decking.io

Update 9

Google kubernetes

Update 10

Redhat have refactored their openshift PAAS to integrate Docker

Project Atomic
Geard

Update 11

A Docker NodeJS lib wrapping the Docker command line and managing it from a json file.

docker-cmd

Update 12

Amazon's new container service enables scaling in the cluster.

Update 13

Strictly speaking Flocker does not "scale" applications, but it is designed to fufil a related function of making stateful containers (running databases services?) portable across multiple docker hosts:

https://clusterhq.com/

Update 14

A project to create portable templates that describe Docker applications:

http://panamax.io/

Update 15

The Docker project is now addressing orchestration natively (See announcement)

Docker machine
Docker swarm
Docker compose

Update 16

Spotify Helios

Update 17

The Openstack project now has a new "container as a service" project called Magnum:

https://wiki.openstack.org/wiki/Magnum

Shows a lot of promise, enables the easy setup of Docker orchestration frameworks like Kubernetes and Docker swarm.

Update 18

Rancher is a project that is maturing rapidly

http://rancher.com/

Nice UI and strong focus on hyrbrid Docker infrastructures

Update 19

The Lattice project is an offshoot of Cloud Foundry for managing container clusters.

Update 20

Docker recently bought Tutum:

https://www.docker.com/tutum

Update 21

Package manager for applications deployed on Kubernetes.

http://helm.sh/

Update 22

Vamp is an open source and self-hosted platform for managing (micro)service oriented architectures that rely on container technology.

http://vamp.io/

Update 23

A Distributed, Highly Available, Datacenter-Aware Scheduler

https://www.nomadproject.io/

From the guys that gave us Vagrant and other powerful tools.

Update 24

Container hosting solution for AWS, open source and based on Kubernetes

https://supergiant.io/

Update 25

Apache Mesos based container hosted located in Germany

https://sloppy.io/features/#features

And Docker Inc. also provide a container hosting service called Docker cloud

https://cloud.docker.com/

Update 26

Jelastic is a hosted PAAS service that scales containers automatically.