Add symfony recommended config to improve php permormance

[eliecharra]

For #38

[eliecharra]

⚠️ With opcache.validate_timestamps, a fpm restart is required for deployment to be effective.

This introduce BC break in this image @jcrombez maybe we should update projects deployed with capistrano under bind mounted volumes in this image to restart fpm to clear opcache cache.

Merge when our deploy strategy were updated only imho

EDIT : parameter removed in 8739e43

[jcrombez]

I don't see this as a BC break really, the normal way of deploying docker container is to restart them so the need to restart fpm shouldn't really be a problem.