An App’s Brief Journey from Source to Image

Pack for the journey

In this tutorial, we’ll explain how to use pack and buildpacks to create a runnable app image from source code.

In order to run the build process in an isolated fashion, pack uses Docker or a Docker-compatible daemon to create the containers where buildpacks execute. That means you’ll need to make sure you have both pack and a daemon installed:

Install pack

Install Docker
or alternatively, see this page about working with podman.

NOTE: pack is only one implementation of the Cloud Native Buildpacks Platform Specification. Additionally, not all Cloud Native Buildpacks Platforms require Docker.

Buildpack base camp

Before we set out, you’ll need to know the basics of buildpacks and how they work.

What is a buildpack?

A buildpack is something you’ve probably used without knowing it, as they’re currently being used in many cloud platforms. A buildpack’s job is to gather everything your app needs to build and run, and it often does this job quickly and quietly.

That said, while buildpacks are often a behind-the-scenes detail, they are at the heart of transforming your source code into a runnable app image.


What enables buildpacks to be transparent is auto-detection. This happens when a platform sequentially tests groups of buildpacks against your app’s source code. The first group that successfully detects your source code will become the selected set of buildpacks for your app. Detection criteria is specific to each buildpack – for instance, an NPM buildpack might look for a package.json, and a Go buildpack might look for Go source files.

What is a builder?

A builder is an image that contains all the components necessary to execute a build. A builder image is created by taking a build image and adding a lifecycle, buildpacks, and files that configure aspects of the build including the buildpack detection order and the location(s) of the run image.

Next stop, the end

Let’s see all this in action using pack build.

Run the following commands in a shell to clone and build this simple Java app.

  1. Clone the samples repo.
git clone
  1. Go to the Java apps sub-directory
cd samples/apps/java-maven
  1. Build the app using pack
pack build myapp --builder cnbs/sample-builder:jammy

NOTE: This is your first time running pack build for myapp, so you’ll notice that the build might take longer than usual. Subsequent builds will take advantage of various forms of caching. If you’re curious, try running pack build myapp a second time to see the difference in build time.

That’s it! You’ve now got a runnable app image called myapp available on your local Docker daemon. We did say this was a brief journey after all. Take note that your app was built without needing to install a JDK, run Maven, or otherwise configure a build environment. pack and buildpacks took care of that for you.

Beyond the journey

To test out your new app image locally, you can run it with Docker:

docker run --rm -p 8080:8080 myapp

Now hit localhost:8080 in your favorite browser and take a minute to enjoy the view.

Take your image to the skies

pack uses buildpacks to help you easily create OCI images that you can run just about anywhere. Try deploying your new image to your favorite cloud!

In case you need it, pack build has a handy flag called --publish that will build your image directly onto a Docker registry. You can learn more about pack features in the documentation.

What about Windows apps?

Windows image builds are now supported!

Windows build guide