Set `sysconfdir` explicitly during build

[tianon]

This allows us to completely remove the php-fpm.conf hacks by having PHP do the right thing by default instead.

There's a lot of overlap here with https://github.com/docker-library/php/pull/1635, which is not incidental -- reviewing that had me scratching my head and feeling sad at my past self. :joy:

So I think that one should probably merge first and then I'll rebase this, but like I said over there, I think this one should wait until the new year too just in case it has some unintended side effect I'm not seeing right now. :see_no_evil: