Step 1: Spin up a VM, and install docker on it,Make sure the docker service is up and running.

Step 2: Log in to the server and open the docker service file /lib/systemd/system/docker.service.

Search for ExecStart and replace that line with the following:

ExecStart=/usr/bin/dockerd -H tcp://0.0.0.0:4243 -H unix:///var/run/docker.sock

Step 3: Reload and restart docker service.

sudo systemctl daemon-reload

sudo service docker restart

Step 4: Validate API by executing the following curl commands. Replace 54.221.134.7 with your host IP.

curl http://localhost:4243/version

curl http://54.221.134.7:4243/version

Once you enabled and tested the API, you can now start building the docker slave image.

Create a Jenkins Agent Docker Image : you can also use this dockerfile vailable at url:

https://github.com/Sonal0409/DevOps_ClassNotes/blob/master/Docker/Jenkins-docker-slave%20dockerfile

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If create your own image than make sure, your image should contain the following minimum configurations to act as a slave.

> sshd service running on port 22.

> Jenkins user with password.

> All the required application dependencies for the build. For example, for a java maven project, you need to have git, java, and maven installed on the image.

Make sure the sshd service is running and can be logged into the containers using a username and password. Otherwise, Jenkins will not be able to start the build process.

Note: The default ssh username is jenkins and the password is also jenkins as per the given Dockerfile. You will have to use these credentials in the below configuration.

Configure Jenkins Server With Docker Plugin

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Step 1: Head over to Jenkins Dashboard –> Manage Jenkins –> Manage Plugins.

Step 2: Under the Available tab, search for “Docker” and install the docker cloud plugin and restart Jenkins.

Step 3: Once installed, head over to Jenkins Dashboard –> Manage Jenkins –>Manage Jenkins –> Manage Nodes and Clouds

Step 4: Select cloud” and fill out the docker host parameters for spinning up the slaves.

Step 5: Under docker, you need to fill out the details like:

Replace “Docker URI” with your docker host IP. For example, tcp://10.128.0.3:4243

You can use the “Test connection” to test if Jenkins is able to connect to the Docker host.

Step 6: Now, from “Docker Agent Template” dropdown, click the “Add Docker template” and fill in the details

Labels – Identification for the docker host. It will be used in the Job configuration. Here we use java-docker-slave

Name: Name of the docker template. Here we use the same name as label ie, java-docker-slave

Docker Image – the image that you created for the slave.

Remote Filing System Root – Home folder for the user you have created. In our case, it’s /home/jenkins

Credentials – click add and enter the SSH username and password that you have created for the docker image.

If you are using my Docker image, the user will be jenkins & password is also jenkins.

By default, the workspace will not be persisted in the host. However,

if you want the workspace to be persistent, add a host volume path under container settings.

For example, if you want the workspace to be available at /home/ubuntu, you can add the volume path as shown below.

/home/jenkins is the path inside the container.

/home/ubuntu:/home/jenkins

Towards the right of the Volumes option, if you click the question mark, it will show you additional volume options as shown below.

Test Jenkins Build Inside a Docker container

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Now that you have the slave configurations ready, we will test the docker agent plugin using a freestyle job.

Create a freestyle job, select “Restrict where this project can be run” option and select the docker host as a slave using the label.

Add a shell build step which echoes a simple “Hello World“

If you have done all the configurations right, Jenkins will spin up a container, builds the project, and destroys the container once the build is done.

First, you will see a pending notification as Jenkins tries to deploy a container on run time and establishes an SSH connection. After a few seconds, your job will start building.

You can check the build logs in your jobs console output as well.