use multi-stage builds in Dockerfile to reduce final image size
use multi-stage builds in Dockerfile to reduce final image size

This commit reduces the uncompressed image size by about 1 GB. A major change is that a python virtual environment is used within the Docker image. This is done so that all installed dependencies are below one directory, and this directory can be copied into the final build stage.

tyarkoni/pliers,tyarkoni/featureX