Laradock changing php-fpm version inside workspace

I am not able to change the php-fpm version inside workspace. I'm trying to install php version 7.1 My docker version is

 Docker version 18.06.1-ce, build e68fc7a

Everytime it installs php version

PHP 7.2.4-1+ubuntu16.04.1+deb.sury.org+1 (cli) (built: Apr  5 2018 08:53:57) ( NTS )

My dockerfile inside workspace looks like as follows:

ARG LARADOCK_PHP_VERSION=7.1

FROM laradock/workspace:2.2-${LARADOCK_PHP_VERSION}

LABEL maintainer="Mahmoud Zalt <mahmoud@zalt.me>"

ARG LARADOCK_PHP_VERSION

# Set Environment Variables
ENV DEBIAN_FRONTEND noninteractive

# Start as root
USER root

###########################################################################
# Laradock non-root user:
###########################################################################

# Add a non-root user to prevent files being created with root permissions on host machine.
ARG PUID=1000
ENV PUID ${PUID}
ARG PGID=1000
ENV PGID ${PGID}

via Chebli Mohamed