Makes the base_url configurable (docker)

[kasisoft]

This change to the configuration makes the base_url for the service configurable using the environment variable CONTEXT_PATH.
This helps in conjunction with reverse proxies.
Standard usage in Docker which opens on localhost:8888

docker run -p 8888:8888 beakerx/beakerx

whereas

docker run -p 8888:8888 -e CONTEXT_PATH=beakerx beakerx/beakerx

would open on localhost:8888/beakerx