Drop namely/protoc-all in favour of a custom Dockerfile?

[kommendorkapten]

When working on https://github.com/sigstore/protobuf-specs/pull/112 I realised that the namely/protoc-all seems to be closed source, and it's very old (Debian Bullseye) which as an example lacks a new Go toolkit (only provides 1.15, released 2021).

It wasn't that hard to hack together a Docker image based on Alpine with rudimentary support for protoc. I think it would be worth exploring the work for adding Java, Ruby and other languages to make sure we know what's get into the image, and are not locked to a closed sourced vendor. As an example, I had to craft a new Docker image with newer Go version to be able to get the JSON schema generation to work.

cc @woodruffw @haydentherapper @znewman01