Fix docker build failure due to upstream golang/dep deprecation unfixed issue

[Chaffelson]

Fixes #71 Uses suggestion from upstream Issue at https://github.com/golang/dep/issues/2223#issuecomment-599872151

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[Chaffelson]

@googlebot I signed it!

[willnorris]

Thanks for the fix! We ended up doing a larger rewrite of the Dockerfile in #82 which also ended up fixing this.