I have a Docker file like the following:
FROM openjdk:8
ADD . /usr/share/app-name-tmp
WORKDIR /usr/share/app-name-tmp
RUN ./gradlew build \
mv ./build/libs/app-name*.jar /usr/share/app-name/app-name.jar
WORKDIR /usr/share/app-name
RUN rm -rf /usr/share/app-name-tmp
EXPOSE 8080
RUN chmod +x ./docker-entry.sh
ENTRYPOINT [ "./docker-entry.sh" ]
The problem is that the final image size is 1.1GB, I know it happens because gradle downloads and stores all the dependencies. What is the best way to remove those unnecessary files and just keep the jar?
Each RUN instruction creates a new layer on top of the existing file system. So the new layer after RUN instruction that deletes you app-name-tmp directory just masks the previous layer containing the downloaded libraries. Hence your docker image still has that size from all the layers built.
Remove the separate RUN rm -rf /usr/share/app-name-tmp instruction and include it in the same RUN instruction that does gradle build as shown below.
RUN ./gradlew build \
mv ./build/libs/app-name*.jar /usr/share/app-name/app-name.jar \
rm -rf /usr/share/app-name-tmp/*
So, your final Dockerfile would be
FROM openjdk:8
ADD . /usr/share/app-name-tmp
WORKDIR /usr/share/app-name-tmp
RUN ./gradlew build \
mv ./build/libs/app-name*.jar /usr/share/app-name/app-name.jar \
rm -rf /usr/share/app-name-tmp/*
WORKDIR /usr/share/app-name
EXPOSE 8080
RUN chmod +x ./docker-entry.sh
ENTRYPOINT [ "./docker-entry.sh" ]
The image built will still add up size from the directory /usr/share/app-name-tmp.