First steps with Docker

Docker has been getting a lot of good press recently. It's a lightweight container environment running on Linux that is like a low-fat version of Vagrant. I thought it was time to try my hand.

Docker is different to Vagrant.

Vagrant Docker
Dev Dev & Production
Persistent Disposable
Many apps Single app
Ruby Bash

The Install

This is a yum/apt online on Linux. But, I'm on Mac. Several Mac tutorials use Vagrant to run Docker.

Alternatively, Docker can be run more transparently using boot2docker:

brew install boot2docker
boot2docker init
boot2docker up
export DOCKER_HOST=tcp://localhost:4243

You should be able to check the version now:

docker version

Your First Container

The steps I've seen over and over again are the ones I'll use here:

  1. Create an base image from one of the main Linux images.
  2. Install some dependencies and your app.
  3. Expose the app on a port.
  4. Save it.

I've created a simple app to start, which can be run from the command line:

git clone https://github.com/alexec/dropwizard-helloworld.git
mvm package

Create Dockerfile in the project directory:

FROM centos

RUN yum -y install java-1.7.0-openjdk-devel.x86_64 

ADD target/dropwizard-helloworld-1.0-SNAPSHOT.jar /
ADD hello-world.yml /

CMD ["java", "-jar", "dropwizard-helloworld-1.0-SNAPSHOT.jar", "server", "hello-world.yml"]

EXPOSE 8080

The Dockerfile is an all in one recipe, and that complete all the steps:

docker build .

When it's complete, it'll print a hash for the image it's built:

Successfully built a3e6a912822c

It's important to understand there's a big difference between an image and a container hash. They look the same, and as commands you might expect to take a container ID sometimes take an image ID.

Start it up:

docker run -i -P a3e6a912822c

Note the -P option, that creates the port forwards from the host OS to the container. Which port is it? It's not the exposed port (8080) as you might expect. Docker chooses a random port for you, and you can see which port it is:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS                     NAMES
67079fc70d31        a3e6a912822c        java -jar dropwizard   9 days ago          Up About a minute   0.0.0.0:49155->8080/tcp   compassionate_tesla

As I'm running Docker within a VM (as I'm on OS-X), I need to set-up a port forward from my computer to the VirtualBox that boot2docker uses:

VBoxManage controlvm boot2docker-vm natpf1 "49155,tcp,127.0.0.1,49155,,49155"

But that's a bit complex! And we need to try and figure out the port each time! Lets do it differently.

docker run -i -p 8080:8080 a3e6a912822c

The -p option maps the container port to a host port.

$ docker ps
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS                    NAMES
c975a2628b03        a3e6a912822c        java -jar dropwizard   2 weeks ago         Up 5 seconds        0.0.0.0:8080->8080/tcp   condescending_bardeen

Now it's running on port 8080. We can easily set-up the forward if we are on OS-X:

VBoxManage controlvm boot2docker-vm natpf1 "8080,tcp,127.0.0.1,8080,,8080"

You can test in your browser: http://localhost:8080/hello-world

Tip: Really useful debugging command (like vagrant ssh):

docker run -i -t a3e6a912822c bash

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