Let Xdebug work without specific settings for different OSes + some slight improvement

[HarkuLi]

Thanks to this repository, I learnt a lot about the combination of nginx and php-fpm.
Here's something I think that could probably be improved when reading the configs.

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What does this implement/fix? Explain your changes.

• Let Xdebug work without specific settings for different OSes
• Improve README
• Fix typo in a comment
• Add missing Makefile targets in .PHONY and reorder it
• Remove unused fpm config: pm.process_idle_timeout
• Use /bin/sh instead for shell commands in the Makefile

Does this close any currently open issues?

No.

Any relevant logs, error output, etc?

No.

Any other comments?

No.

Where has this been tested?

1. Ubuntu 20.04.4 LTS + Docker 20.10.14
2. macOS Big Sur 11.6.6 + Docker Desktop 4.8.2 (Docker 20.10.14)

[sherifabdlnaby]

@HarkuLi Hello, thanks for your PR! I like this enhancement, however I have a small tweak in mind.

I am thinking of keeping the Environment Variable reference in PHP Config, as well as its default value in Dockerfile. This makes the image decoupled from the docker-compose file. So, just in case you wish not to use the already existing docker-compose.yaml file, you won't need to change anything the Dockerfile to make it "configurable".

[HarkuLi]

I have rebased the latest version of main branch because of a conflict.
Additionally,

1. I find that the display of Build Time Arguments table in the latest version README file is broken, so I also rearrange the table in this PR.
2. Because bash shell isn't available in the alpine image, I replace it with sh for shell commands in Makefile.