docs/rootless.md: add instruction for isolating netns
docs/rootless.md: add instruction for isolating netns

Isolating network namespace with `rootlesskit --net=slirp4netns` is
recommended for protecting localhost sockets and abstract sockets on the host.

This is not meaningful for running rootless buildkitd inside a
container, so slirp4netns is not added in Dockerfile.

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc