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-embed.go

@@ -10,6 +10,9 @@ import _ "embed"
 //go:embed VERSION.txt
 var Version string
 
+//go:embed ALPINE.txt
+var AlpineDockerTag string
+
 // GoToolchainRev is the git hash from github.com/tailscale/go that this release
 // should be built using. It may end in a newline.
 //