Use Testcontainers as default for testing

[DavideD]

Now that testcontainers seems to work well on all operating systems, I think we can use it as default. We still need to be able to test on machines not started via docker/podman.

My proposal is to remove -Pdocker (it's going to be the default) and instead have something like -PskipContainers for when we don't want to use Testcontainers or docker. We need to rememeber to update the file podman.md