Changing only environment variable doesn't invalidate build

[corradio]

The caching mechanism checks for file inputs, and might abort a docker build if the inputs haven't changed.
However, other informations might invalidate the build, such as e.g. an environment variable.
The dependency hash should therefore include all information (and not only paths):
https://github.com/tmrowco/brick/blob/master/brick/dockerlib.py#L54-L79