CI: Use a privileged docker container to support FUSE tests

In order to run various tests, we need a working /dev/fuse, and this is
currently only possible in a privileged container.
Github actions workflow doesn't suppport it natively, so we need to
initialize a new one ourself, we just keep it running and execute the
commands when we need them, not to depend on a static image, so that the
github workflow can still be followed as atomic operations.

[1] https://github.com/docker/for-linux/issues/321