hostinfo: set Hostinfo.PackageType for mkctr container builds

Fixes tailscale/corp#21448

Change-Id: Id60fb5cd7d31ef94cdbb176141e034845a480a00
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/packagetype_container.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && ts_package_container
+
+package hostinfo
+
+func init() {
+	linuxBuildTagPackageType = "container"
+}