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I remember the days when packaging and deploying an application meant compiling a binary and running a single command like java -jar app.jar. Today, container images are the dominant distribution format: developers write a Dockerfile, build an image, and deploy that image. Even so, most modern builds still need to produce intermediate artifacts — JARs, WARs, ZIPs, or other binaries — before they are containerized. A custom builder image bridges that gap. It is a purpose-built container image that encapsulates the build toolchain and the build process so the artifact creation happens inside a controlled, reproducible environment.
A side-by-side diagram showing "The original way" (user → terminal → JAR) and "The newer way" (user → DockerFile → container/CLI). It contrasts building/running a JAR with packaging the app using a Dockerfile.
What is a custom builder image?
  • A container image whose single responsibility is to run a defined build workflow.
  • Contains the compiler, build tools, scripts, and any environment configuration needed to produce one or more artifacts.
  • Designed for reproducibility: the same image + same inputs produce identical outputs.
Three red rounded-square buttons with white right-pointing arrows are shown across the image. Each button has a caption underneath: "Plain image with build process logic", "Implements extensions like CICD", and "Sort of like a scratch container image."
Key characteristics
  • Minimal, single-purpose image containing the build toolchain and scripts.
  • Encodes build steps so builds become reproducible and portable.
  • Can be invoked locally, inside CI runners, or by a container platform (e.g., OpenShift, Kubernetes).
  • Extensible: integrate pipeline logic, hooks, or credential handling for private repos and registries.
A custom builder image packages the build environment and process together. It produces artifacts (JAR/WAR/ZIP/etc.) or container images and can be run locally, in CI, or by a container platform.
Typical inputs required by a custom builder image Recommended best practices
  • Keep builder images minimal and focused on the build steps to reduce attack surface and image size.
  • Pin tool and base-image versions to ensure reproducibility across builds.
  • Run builds as non-root where possible and remove unnecessary packages after the build to minimize image bloat.
  • Store credentials securely using platform secrets or a secrets manager; avoid baking credentials into images.
  • Integrate the builder image into CI/CD pipelines so builds are traceable and automated.
Useful links and references

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