Add tests for cache_from functionality

tests/test_cache_from.py

@@ -0,0 +1,24 @@
+"""
+Test that --cache-from is passed in to docker API properly.
+"""
+import os
+import docker
+from unittest.mock import MagicMock, patch
+from tempfile import TemporaryDirectory
+
+def test_cache_from(monkeypatch):
+    FakeDockerClient = MagicMock()
+    cache_from = [
+        'image-1:latest'
+    ]
+    fake_client = MagicMock(spec=docker.APIClient)
+    fake_client.build.return_value = iter([{'stream': 'fake'}])
+
+    with TemporaryDirectory() as d:
+        monkeypatch.chdir(d)
+        from repo2docker.buildpacks import BaseImage
+        for line in BaseImage().build(fake_client, 'image-2', '1Gi', {}, cache_from):
+            assert line == {'stream': 'fake'}
+        called_args, called_kwargs = fake_client.build.call_args
+        assert 'cache_from' in called_kwargs
+        assert called_kwargs['cache_from'] == cache_from