Add PHP 7.4 tests

Jenkinsfile

@@ -49,5 +49,21 @@ pipeline {
 				sh 'phpdbg -qrr -- ./vendor/bin/phpunit --coverage-text --colors=never'
 			}
 		}
+		stage('PHP 7.4') {
+			agent {
+				docker {
+					image 'php:7.4-alpine'
+					args '-u root --privileged'
+				}
+			}
+			steps {
+				sh 'chmod +x ./build/docker_install.sh'
+				sh 'sh build/docker_install.sh'
+				sh 'apk add --no-cache php7-phpdbg'
+				sh 'curl -sS https://getcomposer.org/installer | php'
+				sh 'php composer.phar install --ignore-platform-reqs'
+				sh 'phpdbg -qrr -- ./vendor/bin/phpunit --coverage-text --colors=never'
+			}
+		}
  	}
  }