use alpine:latest, build go 1.16 from source

[jnichols-git]

Working towards a resolution for #691. Since images with Go 1.16.x aren't provided with an updated Alpine base image, the Trickster dockerfile now follows the same steps as the golang image to build from source. You can find an example in the docker-library/golang repository.

This PR doesn't set up any automated processes, just makes the produced Docker image be up-to-date.

[coveralls]

Pull Request Test Coverage Report for Build 7602341868

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 90.991%

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Totals
Change from base Build 879995402:	-0.02%
Covered Lines:	12332
Relevant Lines:	13553

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💛 - Coveralls