Docker hello-world cleanup

[KasperZutterman]

When running the go.sh script, method test_docker_available() runs:
docker run hello-world > /dev/null 2>&1 to test if docker is configuered correctly.
This test container never gets cleaned up, resulting in a bunch of hello-world containers:

[leonstafford]

yikes! Thanks for that, @KasperZutterman! Feel free to send a pull request if you want to submit some code to fix it, else, I'll take a look at it soon

[leonstafford]

mine:

docker ps -a | grep 'hello' | wc -l
88

haha! and that's with some occasional bulk cleanup

[KasperZutterman]

I've created the following pull request in the correct repo.
Hope the solution is sufficient.