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>

cmd/printdep/printdep.go

@@ -19,10 +19,15 @@ import (
 var (
 	goToolchain    = flag.Bool("go", false, "print the supported Go toolchain git hash (a github.com/tailscale/go commit)")
 	goToolchainURL = flag.Bool("go-url", false, "print the URL to the tarball of the Tailscale Go toolchain")
+	alpine         = flag.Bool("alpine", false, "print the tag of alpine docker image")
 )
 
 func main() {
 	flag.Parse()
+	if *alpine {
+		fmt.Println(strings.TrimSpace(ts.AlpineDockerTag))
+		return
+	}
 	if *goToolchain {
 		fmt.Println(strings.TrimSpace(ts.GoToolchainRev))
 	}