Test examples incrementally

currently we test all examples every time and do so outside of bazel

its not really possible (or at least not easy) to make the examples truly hermetic - mostly because they rely on system docker - if we were to switch to `rules_docker` they would still need to parse/translate the `docker-compose` instructions into docker rules

we can however make it almost hermetic - by using a bazel sourced docker-compose, using the required files as inputs and setting an action_env that set the container hash to use for envoy

potentially this can take a massive load off ci as well as making it faster, and also making it easier to (incrementally) test examples locally