Added docker makefile to allow compatibility builds

[cpheps]

Description of the changes

As Jenkins is targeting Go 1.9 for building, I added Makefile-docker.mk with two commands that allow running gofmt and make in a 1.9 environment. This should help mitigate any issues with a dev box not having Go version 1.9.

PR Review Checklist

Author

• [X] add a risk label after carefully considering the "blast radius" of your changes
• [X] describe the intent of your changes in the description. don't just rewrite your code in prose
• [X] assign at least one reviewer

Reviewer

• [ ] review code for readability
• [ ] verify that high risk behavior changes are well tested
• [ ] check license for any new external dependency
• [ ] ask questions about anything that isn't clear and obvious
• [ ] approve the PR when you consider it's good to merge

[cpheps]

Given permissions to merge by @xino12 via slack when #17 was merged.