version: add fallback to fill revision from BuildInfo

If Revision is not set by the linker, fallback to load the revision in
the same format as generated by ./hack/git-meta.

Unfortunately, we have no access to the tags, so we can't have a
fallback for the Version field.

Signed-off-by: Justin Chadwell <me@jedevc.com>

version/version.go

@@ -1,13 +1,47 @@
 package version
 
+import (
+	"runtime/debug"
+	"strconv"
+)
+
+const (
+	defaultVersion = "v0.0.0+unknown"
+)
+
 var (
 	// Package is filled at linking time
 	Package = "github.com/docker/buildx"
 
 	// Version holds the complete version number. Filled in at linking time.
-	Version = "v0.0.0+unknown"
+	Version = defaultVersion
 
 	// Revision is filled with the VCS (e.g. git) revision being used to build
 	// the program at linking time.
 	Revision = ""
 )
+
+func init() {
+	if Revision != "" {
+		return
+	}
+
+	if info, ok := debug.ReadBuildInfo(); ok {
+		revision := ""
+		modified := false
+		for _, setting := range info.Settings {
+			switch setting.Key {
+			case "vcs.revision":
+				revision = setting.Value
+			case "vcs.modified":
+				modified, _ = strconv.ParseBool(setting.Value)
+			}
+		}
+		if revision != "" {
+			Revision = revision
+			if modified {
+				Revision += ".m"
+			}
+		}
+	}
+}