Upgrade terraform syntax for docker tutorial

[jorhett]

This is an opinionated revision of the docker tutorial after working my way through it. If you disagree with some of the changes I'm happy to adjust. I will have more suggestions, but I wanted to see what you thought of this first.

Upgrade the example to Terraform 0.13 (works fine with 0.12 too) Add caveat about logging requiring an environment variable Reduce cognitive load by

• dropping the local backend provider which isn't necessary
• avoiding reuse of values (e.g. 'localhost') as keys

[edwardbartholomew]

@jorhett - Thank you for your interest in kitchen-terraform. I'll give this a try and let you know what I think. Thank you for taking the time to create this PR.

[jorhett]

@edwardbartholomew is this still relevant, or should we close this?

[aaron-lane]

I believe this may have been covered by #460.