dockerTools: format tarsum.go with gofmt

Nearly all Go code on this earth is formatted with gofmt, and it's
somewhat surprising to find a Go file that isn't formatted accordingly.

pkgs/build-support/docker/tarsum.go

@@ -1,24 +1,24 @@
 package main
 
 import (
-    "tarsum"
-    "io"
-    "io/ioutil"
-    "fmt"
-    "os"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"tarsum"
 )
 
 func main() {
-    ts, err := tarsum.NewTarSum(os.Stdin, false, tarsum.Version1)
-    if err != nil {
-        fmt.Println(err)
-        os.Exit(1)
-    }
+	ts, err := tarsum.NewTarSum(os.Stdin, false, tarsum.Version1)
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
 
-    if _, err = io.Copy(ioutil.Discard, ts); err != nil {
-        fmt.Println(err)
-        os.Exit(1)
-    }
+	if _, err = io.Copy(ioutil.Discard, ts); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
 
-    fmt.Println(ts.Sum(nil))
-}
+	fmt.Println(ts.Sum(nil))
+}