Restore phpdbg in fpm images

Hello,

I just came across the PR #1259 and the issue #1257 after wondering why the phpdbg has suddenly disappeared from images like php:7.4-fpm-alpine3.14.

While I understand the point of "It is not suitable to debug fpm processes" the images can still be used with the PHP CLI, and thus, having the need for the phpdbg component. In our case we use the production build to run all tests and generate code coverage, and only install dev dependecies on start of the container. We also use the fpm image to run async workers without fpm started)

The use of phpdbg allows faster tests and not having to instal xdebug specifically for this (which would slow drastically any process).

Leaving it will not impact any existing user as it does not provide (AFAIK) any slowness when not used and will save people time, headache, and allow easier testing.

Thanks

Start by reading PR #1259 and issue #1257, then locate the Dockerfile definition for the php:7.4-fpm-alpine3.14 image. Restore phpdbg for FPM images without changing existing FPM behavior, and verify that the resulting image provides phpdbg for CLI testing and coverage use.