mcrypt extension is not provided with the PHP source since 7.2 , but are instead available through PECL. To install a PECL extension in docker, use pecl install to download and compile it, then use docker-php-ext-enable to enable it:
pecl install mcrypt-1.0.5
docker-php-ext-enable mcrypt
Before the pecl install you may need to install/update the package libmcrypt-dev
apt-get update && apt-get install -y libmcrypt-dev