implemented version command

Signed-off-by: Sam Alba <sam.alba@gmail.com>

cmd/dagger/cmd/root.go

@@ -39,6 +39,7 @@ func init() {
 		plan.Cmd,
 		input.Cmd,
 		output.Cmd,
+		versionCmd,
 	)
 
 	if err := viper.BindPFlags(rootCmd.PersistentFlags()); err != nil {

cmd/dagger/cmd/version.go

@@ -0,0 +1,35 @@
+package cmd
+
+import (
+	"fmt"
+	"runtime"
+	"runtime/debug"
+
+	"github.com/spf13/cobra"
+)
+
+const (
+	defaultVersion = "devel"
+)
+
+// set by goreleaser or other builder using
+// -ldflags='-X dagger.io/go/cmd/dagger/cmd.version=<version>'
+var (
+	version = defaultVersion
+)
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print dagger version",
+	Args:  cobra.NoArgs,
+	Run: func(cmd *cobra.Command, args []string) {
+		if bi, ok := debug.ReadBuildInfo(); ok && version == defaultVersion {
+			// No specific version provided via version
+			version = bi.Main.Version
+		}
+		fmt.Printf("dagger version %v %s/%s\n",
+			version,
+			runtime.GOOS, runtime.GOARCH,
+		)
+	},
+}