How to install Jenkins using Docker by building custom docker image?

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Jenkins is an Open Source automation server used to automate all kinds of tasks related to building, testing and deploying the software. In this automation era, Jenkins plays a major role in set up a Continuous Integration and Continuous Delivery (CI/CD) pipeline. 

CI is the process of automatically the build and testing the code when the code is committed into the version control system. CD is the extension of CI, the process to build, test and deploy into the production environment. 

To start with CI/CD, the first step is to set up the required CI/CD tools and integrate it. So that, we can build and deploy the software seamlessly. In this post, we will see how to setup Jenkins in a containerized environment for creating the CI/CD pipeline. Jenkins is distributed as WAR files, native packages, installers and Docker images.

Steps to install Jenkins using Docker

1. Install docker

Docker installation is the pre-requisite for this Jenkins set up. Visit the official docker site and install the docker based on the platform you are using.

2. Create a Dockerfile to create a custom Jenkins docker image

First, we are referring the jenkins/jenkins:alpine image as the base image and using
an alpine version of the image, which is the slimmest version. And, switching to root
user to install the required packages.
    FROM jenkins/jenkins:alpine

Next, we are installing docker inside the Jenkins image. So, we can run and
execute docker commands in your Jenkins job.
    RUN \
        echo "**** install runtime packages ****"  \
        && apk add --no-cache \
        docker

In order to run the docker commands inside Jenkins job, we have to assign the user
Jenkins to the group docker because all the jobs in Jenkins are executed as a Jenkins
user.
    RUN groupmod -g 801 docker \
        && usermod -aG docker jenkins

If you are going to use ansible for application deployment, then ansible installation
is required.
    RUN apk add --no-cache ansible python3 

Expose the Jenkins home directory and set the default user for this image
    VOLUME /var/lib/jenkins
    USER jenkins

Finally, dockerfile will looks like below

Dockerfile

FROM jenkins/jenkins:alpine
USER root
# set the version label
LABEL maintainer="Navin"
# Update the Environment variables
ENV JENKINS_HOME /var/lib/jenkins
ENV COPY_REFERENCE_FILE_LOG $JENKINS_HOME/copy_reference_file.log
# Install latest Docker CE binaries
RUN \
    echo "**** install runtime packages ****"  \
    && apk add --no-cache \
    docker \
    && echo "**** cleanup ****" \
    && rm -rf \
    /tmp/*
# Modify the UID/GID of user 'jenkins'
# Modify the Jenkins Home directory, also modifying the ownership of it
RUN \
    apk --no-cache add shadow \
    && groupmod -g 800 jenkins \
    && usermod -u 800 jenkins \
    && mkdir "${JENKINS_HOME}" \
    && usermod -d "${JENKINS_HOME}" jenkins \
    && chown jenkins:jenkins "${JENKINS_HOME}"
# Adding the user 'jenkins' to docker group
RUN groupmod -g 801 docker \
    && usermod -aG docker jenkins
RUN \
    echo "**** install ansible and python ****"  \
    && apk add --no-cache ansible python3 
# Expose the files/folder
VOLUME /var/lib/jenkins
# set default user
USER jenkins

3. Create a docker-compose file

Once the dockerfile is ready, we can use docker command to build and run the image or
we can go with docker-compose. Here, we are using docker-compose to build and run the
image.

docker-compose.yml
version: '3'
services:
  jenkins:
    build: .
    container_name: jenkins
    ports:
      -  "8080:8080"
      -  "50000:50000"
    restart: always
    volumes:
      -  "jenkins:/var/lib/jenkins"
      -  "/var/run/docker.sock:/var/run/docker.sock"
volumes:
  jenkins: {}

You have configured the docker-compose file to run the Jenkins container in port 8080.
Port 5000 is mapped for Jenkins slave configuration. As of now, we will leave this
slave configuration part.

To execute the docker-compose commands, docker-compose needs to be installed in the
host machine because it's a separate project which is written in python. For
installation, check this link.
Once it is installed, execute the below command to build and run the container in
detached mode.
    docker-compose up -d

4. Access the Jenkins

By visiting the Jenkins URL in the following format {IP or Domain}:8080.
For eg: www.example.com:8080. You will get Jenkins Unlock page where you have to enter
the admin password which is automatically generated by Jenkins and it is saved under
the /secrets directory of jenkins_home


Once you have entered the password, it will redirect to the plugins installation page.
You can proceed further by selecting the Install Suggested Plugins option. This option
will install around 15+ plugins.



Plugin installation will take 5 to 10 minutes, it depends on the host machine
specification and bandwidth. After plugin installation, you can access the Jenkins
home page and create a job in Jenkins




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