Configure your build and run images

Buildpacks uses a default builder, which consists of all the components necessary to execute a build of your service including both a build and run image:

Build image: Used by the builder image to create the build environment where the buildpacks lifecycle is executed. This is where your application or function is prepared for containerization.
Run image: The base image from which the container image of your service is built. This is the image that hosts your built application or function.

Both images can be customized and extended to suit your needs. For example, you can customize images to add the packages that are required for building your service, or to install system packages so they are available when your service runs.

Before you begin

You use the pack CLI to locally build your service into a container image.

Before you begin

Install Docker Community Edition (CE) on your workstation. Docker is used by pack as an OCI image builder.
Install Pack CLI.
Install the Git source control tool to fetch the sample application from GitHub.

Customize the build and run images

Buildpacks use a builder image to construct your service into a container image. When the build process is complete, your application or function is inserted into a run image. Learn more about the build process at Buildpacks Concepts.

Extending the builder image

To customize the default builder image:

Create a custom builder.Dockerfile from the default builder image. You must specify the tag for the version of the builder that supports your base image. For example, the :v1 base image tag is unsupported by the :google-22 builder tag.

Example:
```
FROM gcr.io/buildpacks/builder
USER root
RUN apt-get update && apt-get install -y --no-install-recommends \
  subversion && \
  apt-get clean && \
  rm -rf /var/lib/apt/lists/*
USER cnb
```
Build your custom builder image from the builder.Dockerfile file:
```
docker build -t BUILDER_IMAGE_NAME -f builder.Dockerfile .
```
Replace BUILDER_IMAGE_NAME with the name that you choose for your custom builder image.
Run the pack build command with your custom builder image to build the new container image of your application or function:
```
pack build SERVICE_IMAGE_NAME --builder BUILDER_IMAGE_NAME
```
Replace:
- SERVICE_IMAGE_NAME with the name that you choose for your application or function image.
- BUILDER_IMAGE_NAME with the name of your custom builder image.

Extending the run image

To customize the default run container image:

Create a custom run.Dockerfile from the default run image. You must specify the tag for the version of the builder that supports your run image. For example, the :v1 run image tag is unsupported by the :google-22 builder tag.

Example:
```
FROM gcr.io/buildpacks/gcp/run
USER root
RUN apt-get update && apt-get install -y --no-install-recommends \
  imagemagick && \
  apt-get clean && \
  rm -rf /var/lib/apt/lists/*
USER 33:33
```
Build your custom run image from the run.Dockerfile file:
```
docker build -t RUN_IMAGE_NAME -f run.Dockerfile .
```
Replace RUN_IMAGE_NAME with the name that you choose for your custom run image.
Run the pack build command with your custom run image to build the new container image of your application or function:
```
pack build SERVICE_IMAGE_NAME --builder gcr.io/buildpacks/builder:v1 --run-image RUN_IMAGE
```
Replace:
- SERVICE_IMAGE_NAME with the name that you choose for your application or function image.
- RUN_IMAGE_NAME with the name of your custom run image.