test: Use ubuntu image

Author: sijis

tests/resources/simple-test.json

@@ -2,9 +2,8 @@
   "builders": [
     {
       "type": "docker",
-      "image": "alpine",
-      "discard": true,
-      "run_command": ["-d", "-i", "-t", "{{.Image}}", "/bin/sh"]
+      "image": "ubuntu",
+      "discard": true
     }
   ]
 }