Use docker image name that is used to run the starter container

[ewoutp]

fixes #52

The recommended commands to start other starters (when using docker) now shows the image that was use to start the container running the first starter.

In this snippet,

docker volume create arangodb2 && \
    docker run -it --name=adb2 --rm -p 8533:8528 -v arangodb2:/data \
    -v /var/run/docker.sock:/var/run/docker.sock arangodb/arangodb-starter:latest \
    --starter.address=192.168.10.11 --starter.join=192.168.10.11

the arangodb/arangodb-starter:latest part is now read from the docker container info of the running container.