runsc: add --restrict-bind-to-loopback flag

[sfc-gh-mkeralapura]

Sandboxed workloads often have no legitimate reason to accept connections on external network interfaces, yet nothing prevents them from calling bind(2) with INADDR_ANY. This flag enforces the principle of least privilege at the kernel level: a process that should only be reachable on loopback cannot accidentally expose itself on a routable interface, even if it attempts to.

It is particularly useful in environments where a sidecar proxy (rather than the application itself) owns the external-facing port and the application should be invisible outside the sandbox.

When enabled, bind(2) returns EACCES for any address that is not a loopback address, including INADDR_ANY (0.0.0.0) and IN6ADDR_ANY (::). Loopback classification is delegated to net.IP.IsLoopback() from the Go standard library. AF_UNIX and other address families are unaffected.

Thread safety: RestrictBindToLoopback is written once during sandbox boot (loader.go:New) before any task goroutines are created, and is read-only thereafter — the same pattern used by AllowSUID and IOUringEnabled.

The flag is only meaningful with sandbox networking (--network=sandbox). With host networking the sentry syscall handlers are not invoked for socket calls, so the check is a no-op.

Tests:

• runsc/config: TestRestrictBindToLoopback verifies flag round-trips
  through RegisterFlags/NewFromFlags/ToFlags.
• test/syscalls/linux/restrict_bind_loopback_test: eight cases covering
  IPv4 INADDR_ANY (rejected), non-loopback unicast (rejected),
  127.0.0.1 / 127.x.x.x (allowed), IPv6 IN6ADDR_ANY (rejected), ::1
  (allowed), ::ffff:127.0.0.1 (allowed), and ::ffff:0.0.0.0 (rejected).
  Tests skip automatically when the flag is not active, so they are safe
  to run on plain Linux or gVisor without the flag.

Assisted-by: Claude Sonnet 4.6

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