Modify proxy test to use mocked FCM API and test it in CI

[reivilibre]

This takes the proxy test introduced in #375 and:

• replaces the nginx forward proxy (which doesn't currently have an AMD64 latest release :S) with a mitmproxy with some hardcoded dummy responses, so we don't need to configure FCM credentials and hit the real FCM API
  • so now you can run the proxy test with minimal faff
• makes curl.sh exit with a failure status if the Sygnal request doesn't give 200 OK
• adds CI code that spawns the Compose setup and then exercises curl.sh, resulting in a success or fail
  • this involves building the dockerfile in more situations than we did before, so caching is now enabled for the docker build
  • we also have to export and import the container image as an 'artifact' to ferry it across jobs

[devonh]

Thanks for doing this, the changes look great!
Just a couple small comments.

[devonh]

Awesome! Hopefully this squashes any future proxy issues from appearing in Sygnal without getting caught in CI.