fix: remove buildpacks from dockerfile test app and add tests for app.json buildpacks

The buildpacks entry in tests/apps/dockerfile/app.json caused
the post-extract trigger to write a .buildpacks file during
Dockerfile-based deploys, breaking builder detection. Remove
the entry and add Go unit tests for getBuildpacks/validBuildpackURL,
a bats integration test for deploying with app.json buildpacks,
and documentation for the new feature.

tests/apps/dockerfile/app.json

@@ -3,11 +3,6 @@
     "dokku-postgres",
     "dokku-redis"
   ],
-  "buildpacks": [
-    {
-      "url": "https://github.com/heroku/heroku-buildpack-nodejs"
-    }
-  ],
   "description": "Used in dokku's test suite",
   "env": {
     "SECRET_TOKEN": {