The idea is simple: instead of configuring specific agents (or "slaves") for every type of project, you set up general-purpose agents with Docker pre-installed.

This means you no longer need to worry about labels for different environments; every agent is equipped to handle multiple project types.

Configuring Permanent Docker Agents

Setting up permanent Docker agents is quite similar to configuring traditional permanent agents, with one key difference: every machine designated as an agent must have Docker installed.

Once Docker is set up on all the machines, the configuration process becomes straightforward. Instead of manually labeling slaves for specific tasks, you use a uniform setup across the board.

After the agents are configured, the Docker image for each build is defined within the pipeline script. For example:

pipeline {
    agent {
        docker {
            image 'openjdk:8-jdk-alpine'
        }
    }
    stages {
        // pipeline steps go here
    }
}

When this pipeline is triggered, the Jenkins agent spins up a container using the openjdk:8-jdk-alpine Docker image, and all the pipeline steps execute inside that container.

This setup ensures consistency across builds, as you always know the exact environment in which your code is running.

Understanding Permanent Docker Agents

If we revisit the scenario where different project types (like Java 7, Java 8, and Python) required separate labeled agents, Docker eliminates that complexity.

Now, each agent is a blank slate with Docker installed, capable of handling any type of project. The pipeline script defines the Docker image required for each specific build.

For example, instead of assigning a Java 8 labeled agent, you specify the appropriate Docker image in the script.

Visually, instead of having multiple agents with different labels, every agent is the same, and Docker does the heavy lifting. This results in streamlined, scalable Jenkins agents that can adapt to any project needs without overcomplicating the setup. By using Docker, you're essentially future-proofing your Jenkins infrastructure while keeping it clean and efficient.

Permanent Docker Agents in Jenkins