[Testing] Test SSH file transfer with images.

[JoeZiminski]

This PR implements SSH tests through a docker image. They key changes in this PR are:

• update code_test_and_deploy.yml to selectively run SSH tests in linux runners. This is because the Windows and macOS runners already are in a container, so it's not possible to run a container from within these containers ('nested containerisation').
• Allow SSH transfer from other ports than the default port 22, by setting an environment variable. This was done for tests as port 22 is already used in GitHub actions.This is added in canonical_configs.py. At the moment this is not documented / exposed to users. In general I don't think there will be much need to SSH from other ports except for 22. But it's possible it will be needed.
• A Dockerfile is added that spins up an ubuntu image with SSH setup so that it can be SSHed into from tests.
• ssh_test_utils.py now contains all machinery to SSH into the docker image.

The rest of this PR is pretty much refactoring. base_transfer.py, test_ssh_file_transfer.py, test_ssh_setup.py just contains the already-existing tests. The new additions is testing the SSH through images not through a configuration where paths etc. were entered into the conftest.py. However, any feedback on these tests is of course welcome.

[niksirbi]

My intermediate report on this:

• The ssh tests failed when locally running on MacOS (M2), with docker running
• The contributing guide should be updated to let contributors know how to handle local ssh tests (either skip them or configure their local machines accordingly).

[niksirbi]

does this mean that on other OSes pytest will run all tests except the ones specified here? That's what we'd want/expect right?