[DnsDock] Fix for image name including forward slash

[wodor]

If docker-compose.yml has an image name that contains forward slash like

web:
    image: someone/image

It results in web.someone/image.docker DNS address and it does not work
/ seems to be an issue here

This PR enables removing everything before slash in container name.
After this change dock-cli ps shows web.image.docker.

[andytson-inviqa]

underscore doesn't work very well in DNS domains

[sroze]

Thanks for this PR! Actually, you should remove the slash and everything before as it's dnsdock's strategy.

[sroze]

Thank you @wodor!