"ADD . /path/in/container" busts docker cache

[rafecolton]

When two docker-builder builds are run in succession without any changes to any of the source files, the build should be entirely cacheable. However running an ADD . /path/in/container still busts the docker cache in many (all?) cases. Presumably this is the same issue as #110. Will need further investigation to determine the exact cause.

/cc @jakedt