Run stage in a Docker Agent

pipeline {     agent none     stages {         stage('Build in Docker') {             agent {                 docker { image 'alpine:latest' }             }             steps {                 sh 'echo "Inside Alpine Container"'                 sh 'cat /etc/os-release'             }         }     } }

but it gave error

Started by user Admininsrtaor
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
WorkflowScript: 6: Invalid agent type "docker" specified. Must be one of [any, label, none] @ line 6, column 17.
                   docker { image 'alpine:latest' }
                   ^

That error means Jenkins isn't recognizing agent { docker { ... } } as a valid declarative agent type.

This happens when the Docker Pipeline plugin isn’t installed/updated or you're using a syntax that your Jenkins version doesn’t support.

Correct Declarative Pipeline Example (requires Docker Pipeline Plugin):
pipeline {
    agent none
    stages {
        stage('Build in Docker') {
            agent {
                docker {
                    image 'alpine:latest'
                }
            }
            steps {
                sh 'echo "Inside Alpine Container"'
                sh 'cat /etc/os-release'
            }
        }
    }
}

If that still throws the same error, you’re on a Jenkins version where Docker agent must be inside agent { docker } without nested block or plugin missing.

First verify the plugin
Dashboard → Manage Jenkins → Manage Plugins → Installed/Available

Search for:
Docker Pipeline
Install & restart Jenkins if missing.

If using an older version, try simplified syntax:
pipeline {
    agent none
    stages {
        stage('Build in Docker') {
            agent {
                docker 'alpine:latest'
            }
            steps {
                sh 'echo "Inside Alpine Container"'
            }
        }
    }
}


If Docker agent still doesn’t work, alternative scripted solution:
node {
    docker.image('alpine:latest').inside {
        sh 'echo "Inside Alpine Container"'
        sh 'cat /etc/os-release'
    }
}


Summary
Cause
Fix
Invalid agent type "docker".        
Install Docker Pipeline plugin
Older Jenkins syntax
Use agent { docker 'img' } or scripted pipeline
Master doesn’t run jobs
Must run on a node with Docker installed