The action causes errors because it is not compatible to the Docker
version used in the runner images. Unfortunately, it doesn't seem
maintained anymore. The action is simple enough, though, so instead of
switching to a fork, we just use "docker run" directly.
Use a Debian-based Docker container to run the unit tests for charon-tkm,
once without and once with TKM running. The container can also be used
locally to run the tests (see comments in the Dockerfile).
Tobias Brunner