Docker: add ALPINE.txt to manage alpine versions

The goal here is to

1. make it so that the number doesn't diverge between the various places
   we had it defined
2. not define the number in corp, only in oss

Signed-off-by: Maisem Ali <maisem@tailscale.com>

version/version_test.go

@@ -0,0 +1,42 @@
+// 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 version
+
+import (
+	"bytes"
+	"os"
+	"testing"
+
+	ts "tailscale.com"
+)
+
+func TestAlpineTag(t *testing.T) {
+	if tag := readAlpineTag(t, "../Dockerfile.base"); tag == "" {
+		t.Fatal(`"FROM alpine:" not found in Dockerfile.base`)
+	} else if tag != ts.AlpineDockerTag {
+		t.Errorf("alpine version mismatch: Dockerfile.base has %q; ALPINE.txt has %q", tag, ts.AlpineDockerTag)
+	}
+	if tag := readAlpineTag(t, "../Dockerfile"); tag == "" {
+		t.Fatal(`"FROM alpine:" not found in Dockerfile`)
+	} else if tag != ts.AlpineDockerTag {
+		t.Errorf("alpine version mismatch: Dockerfile has %q; ALPINE.txt has %q", tag, ts.AlpineDockerTag)
+	}
+}
+
+func readAlpineTag(t *testing.T, file string) string {
+	f, err := os.ReadFile(file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, line := range bytes.Split(f, []byte{'\n'}) {
+		line = bytes.TrimSpace(line)
+		_, suf, ok := bytes.Cut(line, []byte("FROM alpine:"))
+		if !ok {
+			continue
+		}
+		return string(suf)
+	}
+	return ""
+}