Docker is a lightweight alternative to virtual machines like VirtualBox. It uses linux containers (lxc) and cgroups to separate vm instances (called containers in docker). That is a recent Linux features which makes kernel 3.8 a minimum requirement. Containers have their own file system, network interfaces, processes. You can expose container ports outside of container and link containers together.

Images in docker can be build from other images, and containers is a snapshots of the image. This allows you to quickly run several identical images. Containers can be stored after modification, new state will be saved as a diff from original container image.

Although you can log in container and install everything using console it is advised to write a script to generate container - Dockerfile. Docker uses caching and creates a new image after each successful step, next run is much faster because such steps are skipped and last existing image is used. docker build command is used to create an image from Dockerfile.

Docker is not production-ready yet, they target 1.0 somewhere in April 2014. Current version of docker is 0.8. Docker is not running on Windows, MacOS support is very limited. There is an index site for sharing public images and a server for hosting private repositories.

Containers running a single process (web server, memcached) using CMD instruction. Container stops when process stopped. It is possible to use supervisor to run several processes in one container.

Here is my current Dockerfile for installing ruby 1.9.3 and gems specified in Gemfile.

FROM ubuntu
MAINTAINER Eduard Bondarenko <eduard@kobemail.com>

ENV RUBY_VERSION 1.9.3
ENV DEBIAN_FRONTEND noninteractive

# REPOS
RUN apt-get -y update
RUN apt-get -y upgrade
RUN apt-get install -y -q software-properties-common python-software-properties
RUN add-apt-repository -y "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"
RUN add-apt-repository -y ppa:chris-lea/node.js
RUN apt-get -y update

# Ensure UTF-8
RUN locale-gen en_US.UTF-8
ENV LANG       en_US.UTF-8
ENV LC_ALL     en_US.UTF-8

# INSTALL
RUN apt-get install -y -q build-essential openssl libreadline6 \
  libreadline6-dev curl git-core zlib1g zlib1g-dev libssl-dev \
  libyaml-dev libxml2-dev libxslt-dev \
  autoconf libc6-dev ncurses-dev automake libtool bison \
  pkg-config make wget unzip git libmagickwand-dev

# RVM
RUN curl -sL https://get.rvm.io | bash -s stable --auto-dotfiles

ENV PATH /usr/local/rvm/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
RUN rvm requirements

# Install ruby
RUN rvm install ruby-$RUBY_VERSION
RUN /bin/bash -l -c 'rvm use $RUBY_VERSION --default'

RUN apt-get clean

# Add local files to container
ADD . rails

# Forward web server port
EXPOSE 3000

# Change working dir
WORKDIR rails

# Install gems
RUN /bin/bash -l -c 'bundle install'

# Start server
CMD ["./script/rails s thin"]

Read more about docker on site: Docker.io