Container interface: document exposing the host's os-release

Container interface: document exposing the host's os-release

In order to allow applications to detect the host OS version or other
metadata, ask container managers to expose the os-release files as
read-only bind mounts.
For systemd-nspawn, we will also expose ID, BUILD_ID, VERSION_ID and
VARIANT_ID as lowercase environment variables prefixed by the
container_host_ string.