Docker-php-ext-install mcrypt missing folder

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-1.0.4
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

Building on MoiioM's answer, this worked for me using the 7.2-stretch Docker image from PHP

RUN apt-get update && apt-get install -y libmcrypt-dev \
    && pecl install mcrypt-1.0.4 \
    && docker-php-ext-enable mcrypt

To install mcrypt extension you have to make sure you did install libmcrypt-dev which is required.

Try to add:

RUN apt install libmcrypt-dev

before you are trying to install extensions for php.

Update

Try to run first:

docker-php-ext-configure mcrypt

and then

docker-php-ext-install mcrypt