Fix tar-gz reading of Dockerfiles

- Fixes TAR to actually use data
- Fixes buffers for Gunzip to be a proper ArrayBufferView

Fixes #1832

static/js/tar.js

@@ -371,7 +371,7 @@ if (!Array.prototype.some) {
      * @param data The data, in Uint8Array form.
      */
     function Untar(data) {
-        this.data = [];
+        this.data = data;
     }
 
     Untar.prototype.process = function(cb, opt_filter) {