How to install mcrypt on Docker
From PHP manual:
This extension has been moved to the » PECL repository and is no longer bundled with PHP as of PHP 7.4.0.
So in your Dockerfile you have to:
RUN apt-get install libmcrypt-dev RUN pecl install mcrypt-1.0.4 && docker-php-ext-enable mcrypt
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.