Provide a PHAR-based Docker image for easier Installation

[jawira]

Problem

Currently, Composer can be installed within a Dockerfile using a single command:

COPY --from=composer /usr/bin/composer /usr/bin/composer

This is very convenient and simplifies the installation process. I would like to achieve the same with Phing, for example:

COPY --from=phing/phing /usr/bin/phing /usr/bin/phing

However, this is not currently possible because the existing Phing Docker image contains PHP source files instead of a standalone PHAR file.

Proposal

Modify the Phing Docker image to include phing.phar (it will be located at /usr/bin/phing) instead of PHP source files. This would allow users to easily copy and use Phing in their Docker-based workflows, similar to how Composer works.

I am more than willing to implement this change if the feature request is accepted.