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.
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.