Some Docker updates

[JayH5]

I'm considering moving this to its own repo so that it can be updated separately from Vumi itself and Travis can test the container and such things. Anyway, for now here are a few changes.

• Vumi 0.6.8
• Some tweaks to the entrypoint script
• Made the entrypoint script the CMD rather than ENTRYPOINT this means that if you want to pass arguments to the script you have to do CMD ["/app/vumi-entrypoint.sh", "arg1", "arg2"] but it means it's easier to do things like docker run vumi bash and get a shell. I prefer having the entrypoint fixed and updating the command. This will require some small changes to one or two images in mama-ng-dockerconfig.

[hodgestar]

Plus one on making a separate repository for the docker container.

[hodgestar]

:+1: