As you already know, Cloud Native Buildpacks (CNBs) transform your application source code into OCI images that can run on any cloud.

Each buildpack checks the source code and provides any relevant dependencies in the form of layers. Then, buildpack-provided layers are placed atop a runtime base image to form the final application image.

Base image types

A base image is an OCI image containing the base, or initial set of layers, for other images. It is helpful to distinguish between two distinct types of images, Build and Runtime images.

Build image

A build image is an OCI image that serves as the base image for the build environment in which the CNB lifecycle and buildpacks are executed.

A typical build image might determine:

• The OS distro in the build environment
• OS packages installed in the build environment
• Trusted CA certificates in the build environment
• The default user in the build environment

Anatomy of a build image

Typically, a build image may include:

• Shell
• C-compiler
• Minimal operating system distribution, such as Linux utilities that build systems might call out to
• Build time libraries

Runtime image

A runtime image is an OCI image that serves as the base image for the final application image.

A typical runtime image might determine:

• The OS distro or distroless OS in the launch environment
• OS packages installed in the launch environment
• Trusted CA certificates in the launch environment
• The default user in the run environment

Anatomy of a runtime base image

A runtime image may contain:

• No-shell, unless it’s needed by the application
• Runtime libraries, such as Libfreetype
• Runtime platforms, such as python interpreter, which are generally added by buildpacks

For more details on build and runtime images, you can check out the specification

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