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.