Update the debian-base used in PiHole Docker to Buster

Buster has been out for several months so PiHole on Docker should be rebased to use Buster instead. The version of PHP on Stretch (7.0) is no longer supported.

Bumping this

I commented on this here : https://github.com/pi-hole/docker-base-images/pull/14#issuecomment-573166183
The entire thread has a few different takes.

tl;dr we’ll get around to it in a while with proper testing/vetting. Security support for the old OS being dropped will be the impetus to force us to upgrade, we’re in no hurry since we have a big of security patching support time still.