This repo shows how to build Gradle projects with Docker, while taking advantage of things such as build caching and a relatively new Docker feature, Multi-stage builds.

The Dockerfile in this repo does the following, in this order:

• Creates a build container from the Alpine Linux-based Gradle Docker image.
• Sets up a build directory.
• Changes the Gradle home directory so that docker build caches dependencies until the project's Gradle files change.
• Copies in the project's Gradle scripts separately of the source code so that dependencies aren't redownloaded for every single source file change.
• Downloads the project's dependencies.
• Copies the source in and builds the project.

After the project is built:

• A new container is created specifically to host it, based off OpenJDK's Alpine-based JRE 8 image.
• A new user is created inside the container so that the project doesn't run as root.
  • Even though Docker does a pretty good job at isolating containers, it's probably a good idea to refrain from using root privileges unless necessary, just in case the worst happens and someone pulls out a nasty kernel exploit.
• A small launcher script (run.sh) is copied in. This script simply adds all the project's dependencies to the classpath at runtime and runs the project.
• The build output from container #1 is copied in, dependencies and all.
• run.sh is set as the launch command.

The output container is now built and ready to go.

In order for this to work, a few modifications to build.gradle needed to be made.

• A new task ("bundleWithDependencies") has been added to copy the project and dependency JARs to a given output directory.
• A jar section has been added so that the project is built into a JAR file. The output file name in this section needs to match the file name of the JAR in run.sh.
  • The main class attribute in this section also needs to be set to the main class of the project
• An empty task has been added. This exists solely to make Gradle download dependencies and exit afterwards, much like npm install or similar.

(Note: even though the project in this repo is written in Kotlin, the same concept works for Java projects.)