Build from within a git clone by default

Docker does not respect the .dockerignore file when building from a tar.

Instead by default we'll make a local clone into a tmp directory and
build from there. Subsequent builds will reset the clone to match the
checkout.

Compared to building directly in the repo, we'll have reproducible
builds.

Compared to using a git archive:
1. .dockerignore is respected
2. We'll have faster builds - docker can be smarter about caching the
build context on subsequent builds from a directory

To build from the repo directly, set the build context to "." in the
config.

If there are uncommitted changes, we'll warn about them either being
included or ignored depending on whether we build from the clone.

test/configuration/builder_test.rb

@@ -140,7 +140,7 @@ class ConfigurationBuilderTest < ActiveSupport::TestCase
   end
 
   test "context" do
-    assert_equal "-", @config.builder.context
+    assert_equal ".", @config.builder.context
   end
 
   test "setting context" do