Try CI with custom docker file

Dockerfile

@@ -0,0 +1,2 @@
+FROM php:7.4-cli-alpine
+RUN docker-php-ext-install ffi

Jenkinsfile

@@ -1,15 +1,8 @@
 pipeline {
- 	agent none
+ 	agent { dockerfile true }
  	stages {
  		stage('PHP 7.4') {
- 			agent {
- 				docker {
- 					image 'php:7.4-cli-alpine'
- 					args '-u root --privileged'
- 				}
- 			}
  			steps {
- 				sh 'docker-php-ext-install ffi'
  				sh 'apk add --no-cache php7-phpdbg'
  				sh 'curl -sS https://getcomposer.org/installer | php'
  				sh 'php composer.phar install'