Caching Jar dependencies for Maven-based Docker builds

Question

I'm building a Docker image from this Dockerfile:

FROM maven:3.3.3-jdk-8
MAINTAINER Mickael BARON  

ADD pom.xml /work/pom.xml
WORKDIR /work
RUN mvn dependency:go-offline --fail-never

ADD ["src", "/work/src"]
RUN ["mvn", "package"]

With this Dockerfile, I force to download the dependencies before packaging my Java project. Thus, I don't have to redownload the dependencies every time I changed a file from my src directory.

But, there is a problem and this problem is depending on the version of Maven (base image). In fact, the dependencies are downloaded but they are not persisted into the ~/.m2 directory of the container. It's empty. Thus, when I change some source file all the dependencies are redownloaded.

However, I noticed that if I change the version of Maven from the base image (for example FROM maven:3.2.5-jdk-8), it works.

Very strange, isn't it?

Answer 1

There is a new instruction regarding this topic: https://github.com/carlossg/docker-maven#packaging-a-local-repository-with-the-image

The $MAVEN_CONFIG dir (default to /root/.m2) is configured as a volume so anything copied there in a Dockerfile at build time is lost. For that the dir /usr/share/maven/ref/ is created, and anything in there will be copied on container startup to $MAVEN_CONFIG.

To create a pre-packaged repository, create a pom.xml with the dependencies you need and use this in your Dockerfile. /usr/share/maven/ref/settings-docker.xml is a settings file that changes the local repository to /usr/share/maven/ref/repository, but you can use your own settings file as long as it uses /usr/share/maven/ref/repository as local repo.