updated docker files to use go modules

[sarge]

❤ THANKS FOR HELPING OUT :D

Proposed change

Updates docker files.
closes #103

Types of changes

What types of changes is this pull request introducing to flottbot? Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

You can fill this out after creating your PR. Put an x in the boxes that apply

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Pull Request Test Coverage Report for Build 219

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 39.444%

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Totals
Change from base Build 217:	0.0%
Covered Lines:	781
Relevant Lines:	1980

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💛 - Coveralls

[sjqnn]

If we're ripping out dep from all of the image flavored Dockerfiles, we may as well remove it from the base Dockerfile as well. /~https://github.com/target/flottbot/blob/master/docker/Dockerfile#L8

[sarge]

@sjqnn Updated that one too:)

[JordanSussman]

LGTM besides the one minor Makefile update.

[JordanSussman]

I just resolved the merge conflicts and addressed the one issue for the Makefile. Anyone else have objections to getting this merged?

[delta592]

looks good