Fixed usage instructions for DNS docker appliance

[eantowne]

• Fixed some typos and clarified portions of the "Usage Instructions"
• Added instructions to fix interface to bind to "interface=eth0"

Before submitting a pull request, please check the following.

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When updating an existing appliance:

• [-] The new version is on top.
• [-] The filenames in the "images" section are unique, to avoid appliances / version overwriting each other.

• [x] If you forked the repo, running check.py doesn't drop any errors for the updated file.

  When creating a new appliance:

• It's tested locally, i.e.
  • [ ] You dragged an instance into a project on your box, got it installed (if necessary), and did some basic network checks (ping, UI reachable, etc.).
  • [ ] GNS3 VM can run it without any tweaks.
  • [ ] The device is in the right category: router, switch, guest (hosts), firewall
  • [ ] You filled in as much info as possible (checks the schemas and other appliance files for some guidance).
• [ ] When adding a container: it builds on Docker Hub and can be pulled.
• [ ] The filenames in the "images" section are unique (to avoid appliances and/or versions overwriting each other).
• [ ] If you forked the repo, running check.py doesn't drop any errors for the new file.
• [ ] Optional: a symbol has been created for the new appliance.