hostinfo: set Hostinfo.PackageType for mkctr container builds

Fixes tailscale/corp#21448

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

hostinfo/hostinfo_linux.go

@@ -159,7 +159,14 @@ func linuxVersionMeta() (meta versionMeta) {
 	return
 }
 
+// linuxBuildTagPackageType is set by packagetype_*.go
+// build tag guarded files.
+var linuxBuildTagPackageType string
+
 func packageTypeLinux() string {
+	if v := linuxBuildTagPackageType; v != "" {
+		return v
+	}
 	// Report whether this is in a snap.
 	// See https://snapcraft.io/docs/environment-variables
 	// We just look at two somewhat arbitrarily.

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"
+}