runtime: add support for readonly sandbox bindmounts

If specified, sandbox_bind_mounts identifies host paths to be
mounted (ro) into the sandboxes shared path. This is only valid
if filesystem sharing is utilized.

The provided path(s) will be bindmounted (ro) into the shared fs directory on
the host, and thus mapped into the guest. If defaults are utilized,
these mounts should be available in the guest at
`/var/run/kata-containers/shared/containers/sandbox-mounts`

These will not be exposed to the container workloads, and are only
added for potential guest-services to consume (example: expose certs
into the guest that are available on the host).

Fixes: #1464

Signed-off-by: Eric Ernst <eric.g.ernst@gmail.com>

src/runtime/virtcontainers/sandbox.go

@@ -116,6 +116,9 @@ type SandboxConfig struct {
 
 	DisableGuestSeccomp bool
 
+	// SandboxBindMounts - list of paths to mount into guest
+	SandboxBindMounts []string
+
 	// Experimental features enabled
 	Experimental []exp.Feature