Lets look at official manual for php docker image Section PHP Core Extensions
For example, if you want to have a PHP-FPM image with iconv, mcrypt
and gd extensions, you can inherit the base image that you like, and
write your own Dockerfile like this:
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
Rember, you must install dependencies for your extensions manually. If
an extension needs custom configure arguments, you can use the
docker-php-ext-configure script like this example. There is no need to
run docker-php-source manually in this case, since that is handled by
the configure and install scripts.