Can't install php5.6-gd in docker container

I want to create a container with php5.6-fpm but when I run it, PHP there doesn't have such needed extensions as e.g. gd and some others needed for wordpress. I want to install it manually inside the container fpm. docker exec -it fpm bash It turns out that Debian 9 installed there and I can't install php5.6-gd for E: Package 'php5.6-gd' has no installation candidate! I tried adding ondrej repository, but it's not helpful at all.

Is there any other ways to get container with php5.6-fpm installed and add some extension to it?

Here you can find working Dockerfile for what you need.

Relevant part:

FROM php:5.6-fpm
(...)
RUN apt-get update && \
apt-get install -y --no-install-recommends \
    curl \
    libmemcached-dev \
    libz-dev \
    libpq-dev \
    libjpeg-dev \
    libpng12-dev \
    libfreetype6-dev \
    libssl-dev \
    libmcrypt-dev

(...)
# Install the PHP gd library
RUN docker-php-ext-install gd && \
    docker-php-ext-configure gd \
        --enable-gd-native-ttf \
        --with-jpeg-dir=/usr/lib \
        --with-freetype-dir=/usr/include/freetype2 && \
    docker-php-ext-install gd