安装ldap扩展Docker

Hey all I am trying to instal ldap extension on a Docker php:5.6-fm images i need ldap for my project.

I have tired to install the extension through Dockerfile like so:

RUN apt-get install php5-ldap -y

get this error:

The LDAP PHP extension is not enabled.

I have also find some 'suggestion' online like so:

RUN \
    apt-get update && \
    apt-get install libldap2-dev -y && \
    rm -rf /var/lib/apt/lists/* && \
    docker-php-ext-configure ldap --with-libdir=lib/x86_64-linux-gnu/ && \
    docker-php-ext-install ldap

get this error:

An exception occured in driver: could not find driver

am i doing something wrong...? how do i install ldap on a docker image so that i can use it in my project...?