Add cgi-fcgi to fpm images to allow healthchecks

[RoepLuke]

By default this image does not have a healthcheck defined.
If one wants to do so there are a few options:

• tcp connect test inside the container (no advanced information)
• using information from occ status (information only semi-relevant for healthchecking php-fpm)
• HTTP/S request via webserver/reverseproxy container (tests multiple containers instead of only this one)
• request php-fpm status page via the cgi-fcgi tool (tool missing, information may be misinterpreted/misleading)
• etc.

Currently the fpm images do not ship the cgi-fcgi binary.
Implementing a healthcheck based on it thus requires building your own image that includes this binary.

Such healthchecks could be:

• use serverinfo for docker healthcheck #676 (comment)
• https://github.com/renatomefi/php-fpm-healthcheck

Even with the binary included I wouldn't ship a healthcheck by default because of the issues outlined above.
However I do think it should be possible to use cgi-fcgi without having to build your own image - if there are no greater reasons not to include such a tool inside this image.

Thank you very much

[J0WI]

Explicit health checks are not added to official images: https://github.com/docker-library/faq#healthcheck