Jenkinsfile for parallel Docker images (Python)

Jenkinsfile

#!groovy
def docker_images = ["python:3.5.5", "python:3.6.4", "python:3.6.5"]

def get_stages(docker_image) {
    stages = {
        docker.image(docker_image).inside {
            // If I use this variable as the Virtual Env Folder,
            // I get "permission denied" messages from pip, since it tries
            // to install into /usr/local/... instead of the pyenv folder!
            // def PYTHON_VENV = docker_image.replaceAll('[:.]', '') + 'venv'
          
            // This is the name of the folder where the Virtual Env is created
            // However, this is shared between docker images and is not isolated!
            def PYTHON_VENV = 'venv'

            stage("${docker_image}") {
                echo "Running in ${docker_image}"
            }
            stage("Prepare") {
                sh "rm -rf ${PYTHON_VENV}"
                sh "python -m venv ${PYTHON_VENV}"
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    pip install -U pip setuptools wheel
                """
            }
            stage("Build") {
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    make
                """
            }
            stage("Install Dependencies") {
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    make dependencies
                """
            }
            stage('Test') {
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    make clean
                    make test
                """
                junit 'junit.xml'
            }
            stage("Install") {
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    make install
                """
            }
            stage('Docs') {
                sh """
                    . ${PYTHON_VENV}/bin/activate
                    make doc-dependencies
                    cd docs
                    export MPLBACKEND="agg"
                    make html
                """
            }

        }
    }
    return stages
}

node('master') {
    checkout scm

    def stages = [:]
    for (int i = 0; i < docker_images.size(); i++) {
        def docker_image = docker_images[i]
        stages[docker_image] = get_stages(docker_image)
    }

    parallel stages

}