Adding more php-extension modules

[catalystfd]

Hi all, wanted to check if you were open to PR's adding another php-extension module, specifically in this case the imap module.

This is used in a few of our plugins and would be useful to have by default in the upstream container.

Roughly, in each of the supported environments

RUN apt update && apt install -y libc-client-dev libkrb5-dev && rm -r /var/lib/apt/

RUN docker-php-ext-configure imap --with-kerberos --with-imap-ssl && docker-php-ext-install imap

will attempt to integrate with the setup script that manages the extensions.