Add LDAP support to PHP 5.6 workspace

Issue #1296:
PHP 7.0 and 7.1 workspaces support adding the PHP LDAP module. The
PHP 5.6 workspace file seems to have missed this for some reason.

workspace/Dockerfile-56

@@ -72,6 +72,19 @@ RUN if [ ${INSTALL_SOAP} = true ]; then \
   apt-get -y install libxml2-dev php5.6-soap \
 ;fi
 
+#####################################
+# LDAP:
+#####################################
+
+ARG INSTALL_LDAP=false
+ENV INSTALL_LDAP ${INSTALL_LDAP}
+
+RUN if [ ${INSTALL_LDAP} = true ]; then \
+    apt-get update -yqq && \
+    apt-get install -y libldap2-dev && \
+    apt-get install -y php5.6-ldap \
+;fi
+
 #####################################
 # IMAP:
 #####################################