hostinfo,build_docker.sh,tailcfg: more reliably detect being in a container (#13826)

Our existing container-detection tricks did not work on Kubernetes,
where Docker is no longer used as a container runtime. Extends the
existing go build tags for containers to the other container packages
and uses that to reliably detect builds that were created by Tailscale
for use in a container. Unfortunately this doesn't necessarily improve
detection for users' custom builds, but that's a separate issue.

Updates #13825

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

hostinfo/hostinfo_linux_test.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux && !android
+//go:build linux && !android && !ts_package_container
 
 package hostinfo
 
@@ -34,3 +34,9 @@ remotes/origin/QTSFW_5.0.0`
 		t.Errorf("got %q; want %q", got, want)
 	}
 }
+
+func TestInContainer(t *testing.T) {
+	if got := inContainer(); !got.EqualBool(false) {
+		t.Errorf("inContainer = %v; want false due to absence of ts_package_container build tag", got)
+	}
+}