Docker php:fpm—install php extensions - php

Question

I am using the official php:fpm docker image as base for my application container, so the Dockerfile starts like so:
FROM php:fpm
Later in the file I would like to have something like that:
RUN apt-get install -y \
php7.0-gd
But that tells me:
E: Unable to locate package php7.0-gd
E: Couldn't find any package by regex 'php7.0-gd'
»Bashing« into the container using docker exec -it <name> /bin/bash and executing: apt-cache search php | grep gd yields:
php5-gdcm - Grassroots DICOM PHP5 bindings
php5-vtkgdcm - Grassroots DICOM VTK PHP bindings
php5-gd - GD module for php5
So since that is a debian (yessie) based image, only the old php5 packages are available and php7 is installed by some tricky script in the php:fpm dockerfile and it seams that all extensions are compiled within the used php executable.
How can I install more extensions in this scenario?

Answer 1

Quoting https://hub.docker.com/_/php/
How to install more PHP extensions
We provide the helper scripts docker-php-ext-configure, docker-php-ext-install, and docker-php-ext-enable to more easily install PHP extensions.
To install PHP with iconv, mcrypt and GD, they provide the following example:
FROM php:7.0-fpm
RUN apt-get update && apt-get install -y \
libfreetype6-dev \
libjpeg62-turbo-dev \
libmcrypt-dev \
libpng-dev \
&& docker-php-ext-install -j$(nproc) iconv mcrypt \
&& docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ \
&& docker-php-ext-install -j$(nproc) gd
Please refer to the Dockerhub page for more details.