Test image build fails from symlinked folder

[jojje]

When trying to build an image from a directory that is a symlink the test image build step fails.

INFO: Found Dockerfile.test, building test image tomologic/fluentd_shipper:v0.2.0-test

unable to prepare context: The Dockerfile (/Users/jonas/dev/salt/saltcheck/image_src/Dockerfile.test) must be within the build context (.)
exit status 1

To reproduce:

1. ln -s <path to real project directory> myproject
2. cd myproject
3. wrench build -r