PHP arm64 support, buster variants for back compat

[Chuxel]

As described in #558 (comment), the situation for Docker on M1 macs is not ideal since both debian:buster nor ubuntu:focal experience segmentation faults due to an issue in libss1.1. It seems unlikely that buster will be patched now that bullseye is out (and buster therefore gets security fixes only).

Therefore this PR:

1. Adds arm64 builds for Debian bullseye
2. Adds buster variants that are x86 only for backwards compat.
3. Defaults to bullseye since this is what the upstream php image has done.
4. Makes a few minor edits to the python definition README.md and definition-manifest.json.

This also adds php:<PHP VESION>-apache-<OS> tags because of the tagging scheme used in the php upstream image, but is really only there to enable base.Dockerfile to function during build.

//cc: @joshspicer @2percentsilk @bamurtaugh @chrmarti