build container image for testing

[rfc2119]

As stated in #1 , testing should be done against the real server in action (that is, for services that can be self-hosted). A container image should be built containing the currently supported services and pushed to an image container registry.

[m-242]

After discussion and advice asking, it has been determined that we should use two sort of tests, integration tests running only on PRs on master, and maybe dev, that work exactly as described in the issue's initial comment, and unit testing, that only compare forged requests with what they should look like, according to the file hosting service's API last time tests were updated. These should be much faster, and hence run on every push.