Docker: In Dockerfile, copy files temporarily, but not for final image

Question

I have a Java service I'd like to package, and the only thing the final docker image needs is the JAR file and a config file. However, I need to run my gradle command first to build the JAR, but I don't want all the things that gradle uses to be in the result docker image.

Here's my current DockerFile:

RUN apt-get update && apt-get install -y openjdk-7-jdk
COPY . /
RUN ./gradlew shadowJar
CMD ["java", "-jar", "service/build/libs/service.jar", "server", "service/service.yml"]

You can see I have to COPY everything first so that I can run ./gradlew (otherwise it says the command cannot be found). But in the end, all I need is the service.jar and service.yml files.

I'm probably missing something, but how I can make everything available during the ./gradlew build step, but only have the result image include the service.jar and service.yml.

Answer 1

Docker introduced so called "multi stage builds". With them, you can avoid having unnecessary files in your image. Your Dockerfile would look like this:

FROM ubuntu AS build
COPY . /src
# run your build

FROM ubuntu
RUN mkdir -p /dist/
COPY --from=build /src/my.object /dist/

The principle is simple. In the first FROM you name your build. In the second FROM you can use the COPY --from=-parameter to copy files from your first build to your second. The second build is the one which results in an usable image later.

If you want to test the results of your build instead the resulting image, you can use docker build --target build myimage:build .. The resulting image only includes the first FROM in your Dockerfile.

For more information: https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds

Answer 2

There is an experimental build flag --squash to summarize everything into one layer.

For example, docker build --squash -t <tag> .

The documentation https://docs.docker.com/engine/reference/commandline/image_build/#options.

The Github discussion https://github.com/moby/moby/issues/332.

How to enable experimental features in the daemon? https://github.com/docker/docker-ce/blob/master/components/cli/experimental/README.md