Docker build not picking up built jar?
It seems to me that your Dockerfile should be looking like this:

FROM ~~container_on_private_registry~~
USER root
WORKDIR /root
ARG jenkins_jar_file=jenkins.jar
ENV jenkins_jar_file ${jenkins_jar_file}
RUN mkdir -p /root/app
COPY ${jenkins_jar_file} /root/app/app.jar
ENTRYPOINT ["java", "-jar", "/root/app/app.jar"]


Explanation:

We tell the Dockerfile to let Docker know, that it expects a variable named jenkins_jar_file to be passed to it during the build. Subsequent lines can reference that variable with a dollar notation.

Also when you build the image, use --no-cache so it will re-build all the layers of the image.