Also run tests against docker container

## Currently

Almost all integration tests run against local server instances (`server.listen()` on the main process) of the proxy and the mock.

This means that problems specific to the Docker image build are not caught by the tests. Issues like #401 will go unnoticed.

## This change will...

* Build the Docker container before the tests are run.
* Re-run integration tests also against the Docker container of openapi-cop.