CI: test `DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=slirp4netns`

[AkihiroSuda]

VXLAN is known to work with DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=builtin (default), but not tested yet with DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=slirp4netns.

Eventually it has to be tested with pasta too:

• pasta: support UDP forwarding rootlesskit#383