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

[Chaffelson]

Fixes #71
Uses suggestion from upstream Issue at golang/dep#2223 (comment)

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[Chaffelson]

@googlebot I signed it!

[dockerzhang]

it works for me.

[willnorris]

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