symfonyAppPipeline: add build-symfony-image hook

vars/symfonyAppPipeline.groovy

@@ -109,6 +109,8 @@ def buildDockerImage(String baseImage) {
     def addLetsEncryptCA = libraryResource 'com/cadoles/common/add-letsencrypt-ca.sh'
     writeFile file:'add-letsencrypt-ca.sh', text:addLetsEncryptCA
 
+    hook('build-symfony-image')
+
     def safeJobName = URLDecoder.decode(env.JOB_NAME).toLowerCase().replace('/', '-').replace(' ', '-')
     def imageTag = "${safeJobName}-${env.BUILD_ID}"
     return docker.build("${imageName}:${imageTag}", '.')