Developing With Docker - Building Patterns

This is my 4th post on developing with Docker. In this post deals with some common ways you can build Docker images.

I'm going to assuming you've already packaged your application (e.g. into a JAR).

There are several choices: scratch + binary, language stack, and distro + package manager.

As usual, you can find the source code on Github.

Scratch + Binary

Scratch is the most basic base image, it does not contain any files or programs at all. You must build a standalone binary application if you are going to use this. Lets see an example.

Firstly, we'll build a standalone binary application using Docker. Create an empty directory, and then create main.go with the following content:

package main

import "fmt"

func main() {
    fmt.Println("Hello World")
}

Compile the application:

docker run --rm -ti -v $(pwd):/go/src/myapp google/golang go build myapp

Create a Dockerfile for the application:

FROM scratch
ADD myapp /
CMD ["myapp"]

Finally build and run the image:

docker build -t myapp:1 .
docker run --rm -ti myapp:1

You'll see "Hello World" printed in your terminal.

Advantages:

As the image contains just one file it has a very low attack surface.
Typically small images, and runtime memory usage.

Disadvantages

Only suitable for applications that can be packaged as standalone binaries
No language runtime, so larger applications will take more disk-space as they do not share image layers.
No built in diagnostics tools, such as curl.

Language Stack

Docker provide a number of pre-built base images for the runtime for common languages.

Lets look an example. Create a new empty directory and in it, Main.java:

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Compile this application using the Java Development Kit:

docker run --rm -ti -v $(pwd):/myapp -w /myapp java:8-jdk javac Main.java

Create the following Dockerfile (which contains the Java Runtime Environment):

FROM java:8-jre
ADD Main.class /
CMD ["java", "-cp", "/", "Main"]

Finally, you can build and run this image:

docker build -t myapp:1 .
docker run --rm -it myapp:1

Advantages

Fast to deploy, once the base image is downloaded.
If you use the same base image for many apps, the additional layer needed is tiny.

Disadvantages

Base image must be downloaded.
You'll need to consider how you re-build and re-deploy images if you find a security vulnerability in the base.

Distro + Package Manager

If you want to build an image that is not on a supported language stack, then you'll need to roll your own, starting with a distro, and then using a package manager to add any dependencies you need.

Linux always contains a package manager, we can build a similar images to the Java base image used above by starting with Ubuntu:

FROM ubuntu:15.10

RUN apt-get update && apt-get install --no-install-recommends -y openjdk-8-jre

ADD Main.class /

CMD ["java", "-cp", "/", "Main"]

As per the last examples, build and run this base image:

docker build -t myapp:1 .
docker run --rm -it myapp:1

Advantages

Flexible, can build any application.
Can put multiple applications into a single image (using systemd).

Disadvantages

Longer build time.
You'll need to consider how you re-build and re-deploy images if you find a security vulnerability in the base.

I hope this gives a few ideas about the various pros and cons of different ways to build images. I have a number of other posts on Docker.

Developing With Docker - Building Patterns

Scratch + Binary

Language Stack

Distro + Package Manager

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Alex Collins

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