Jenkins Image, using Docker-in-Docker

Jenkins-Dockerfile

# We need Ruby to do a lot of stuff; let's install it here
# Using buster because that's what the Jenkins build is based on.
FROM ruby:2.7-buster as ruby
COPY --from=ruby / /

FROM jenkins/jenkins:lts-jdk11

MAINTAINER Robert Watkins

USER root
RUN mkdir /var/log/jenkins
RUN mkdir /var/cache/jenkins
RUN chown -R  jenkins:jenkins /var/log/jenkins
RUN chown -R  jenkins:jenkins /var/cache/jenkins

# We also need to install Docker in Docker, because despite the fact that running Docker containers
# as part of Jenkins jobs is a standard practice, it is yet to become part of the standard
# installation process in the Jenkins image.
RUN apt-get update
RUN apt-get install -y apt-transport-https ca-certificates curl gnupg2 software-properties-common
RUN curl -fsSL https://download.docker.com/linux/$(. /etc/os-release; echo "$ID")/gpg | apt-key add -
RUN add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/$(. /etc/os-release; echo "$ID") $(lsb_release -cs)  stable"
RUN apt-get update

# NB: The 996 is the group id for the docker group on the ECS server (Amazon Linux 2).
# Using the same group ID provides all of the necessary OS-level permissions and means
# we don't need to do any special hacks to get Docker running on the build server.
RUN groupadd -g 996 docker && usermod -a -G docker jenkins
RUN apt-get install -y docker-ce


# Okay - now for the Jenkins configuration.
USER jenkins

ENV JAVA_OPTS="-Xms1536m -Xmx1536m"

# "For 2.x-derived images, you may also want to" - we're 2.0 dervied, so we want this
RUN echo 2.0 > /usr/share/jenkins/ref/jenkins.install.UpgradeWizard.state

# Put the log file into the log directory, which will be in the data volume
# Move the WAR out of the persisted jenkins data dir
# add a prefix to the URL, making it easier to route between various apps.
ENV JENKINS_OPTS="--logfile=/var/log/jenkins/jenkins.log --webroot=/var/cache/jenkins/war --prefix=/jenkins"