Add a dedicated console image

[FlorianHockmann]

JanusGraph comes with a Gremlin Console pre-configured for JanusGraph that can also be used via the Docker image as described in the README.md.

This could be simplified with a dedicated Docker image for the Gremlin Console as suggested by @farodin91 here. Users wouldn't need to explicitly provide ./bin/gremlin.sh as an argument and the image size would probably also be much smaller.