Dockerfile: add test that build-env Alpine version matches go.mod

So things like #5660 don't happen in the future.

Change-Id: I01234f241e297d5b7bdd18da1bb3cc5420ad2225
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version_test.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tailscaleroot
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestDockerfileVersion(t *testing.T) {
+	goMod, err := os.ReadFile("go.mod")
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := regexp.MustCompile(`(?m)^go (\d\.\d+)\r?$`).FindStringSubmatch(string(goMod))
+	if m == nil {
+		t.Fatalf("didn't find go version in go.mod")
+	}
+	goVersion := m[1]
+
+	dockerFile, err := os.ReadFile("Dockerfile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantSub := fmt.Sprintf("FROM golang:%s-alpine AS build-env", goVersion)
+	if !strings.Contains(string(dockerFile), wantSub) {
+		t.Errorf("didn't find %q in Dockerfile", wantSub)
+	}
+}