Docker-in-docker breaks paths to test results

The changes for detecting docker mounts (issue #24) works fine when the Azure Pipelines agent is running on the host, and the docker container is just running the tests.
However, if you want to use docker-in-docker or docker siblings, the Azure Pipelines agent will be running in a docker container, so it is not expecting the file path to be converted for the host.
https://jpetazzo.github.io/2015/09/03/do-not-use-docker-in-docker-for-ci/

Unfortunately, I can't just disable it with `pytest --no-docker-discovery` because it won't print `##vso[results.publish type=NUnit...` if that option is used.
https://github.com/tonybaloney/pytest-azurepipelines/blob/02e6685f8a20e52fda7242dd01f6ca1eabd368cd/pytest_azurepipelines.py#L88

You don't actually need to run two docker containers to cause the agent to fail to find the test results file. If you just use `docker volume create` and mount it over `/azp`, then `apply_docker_mappings()` will modify the paths.