gradle

First, create an empty directory for the project:

$ mkdir gradle-demo && cd gradle-demo

Then, initialize a new Gradle application project using the Docker image:

$ docker run --rm -v "$(pwd)":/build registry.ghost-prod.alphabravo.io/ghost-base/gradle:<tag> \
  init --type java-application --dsl kotlin --test-framework junit \
  --project-name gradle-demo --package org.example

You now have a complete Gradle project to use with the examples in this guide.

Build a project

You can build a local project by bind mounting your project directory into the container and running the build task.

$ docker run --rm \
  -v "$(pwd)":/build \
  registry.ghost-prod.alphabravo.io/ghost-base/gradle:<tag> \
  build

Build and package application artifacts

You can create executable JAR files by running the assemble task. This task compiles your code and packages it into JAR or WAR files, depending on your project configuration.

$ docker run --rm \
  -v "$(pwd)":/build \
  registry.ghost-prod.alphabravo.io/ghost-base/gradle:<tag> \
  clean assemble

Multi-stage Dockerfile builds

Ghost hardened images for Gradle are build-only tools. They contain no runtime variants because Gradle builds applications but doesn't run them. You must use multi-stage Dockerfiles to copy build artifacts to appropriate runtime images.

The following example uses the Ghost hardened image for Eclipse Temurin as the runtime stage.

# syntax=docker/dockerfile:1
# Build stage - Gradle DHI for building
FROM registry.ghost-prod.alphabravo.io/ghost-base/gradle:<tag> AS build

# Copy Gradle files for better caching (working directory is /build by default)
COPY settings.gradle.kts ./
COPY app/build.gradle.kts app/
COPY gradlew gradlew.bat ./
COPY gradle/ gradle/
COPY app/src app/src

# Build the application
RUN --mount=type=cache,target=/home/gradle/.gradle \
    gradle clean assemble --no-daemon

# Runtime stage - JRE for running the application
FROM registry.ghost-prod.alphabravo.io/ghost-base/eclipse-temurin:<tag> AS runtime

WORKDIR /app
COPY --from=build /build/app/build/libs/*.jar app.jar

EXPOSE 8080
ENTRYPOINT ["java", "-cp", "app.jar", "org.example.App"]

Non-hardened images vs. Ghost hardened images

The following table summarizes the key differences between standard Docker Gradle images and Docker hardened Gradle images.

Image variants

Ghost hardened images come in different variants depending on their intended use. The Gradle Ghost hardened images are intended for build-time only. All variants include dev in the tag name and are designed for use in build stages of multi-stage Dockerfiles.

Migrate to a Ghost hardened image

Switching to the hardened Gradle image does not require any special changes. You can use it as a drop-in replacement for the standard Gradle image in your existing workflows and configurations. Note that the entry point for the hardened image may differ from a standard image, so ensure that your commands and arguments are compatible.

Migration steps

1. Update your image reference. Replace the image reference in your Docker run command, Dockerfile, or Compose file:
   • From: gradle:<tag>
   • To: registry.ghost-prod.alphabravo.io/ghost-base/gradle:<tag>
2. All your existing environment variables, volume mounts, and network settings remain the same.

Troubleshoot migration

The following are common issues that you may encounter during migration.

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Ghost hardened images is to use Docker Debug to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Ghost hardened images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Ghost hardened images and update your Dockerfile if necessary.