Feature/make docker image smaller

[aorfanos]

Changes:

• updated Go version to 1.17
• used distroless static image

Size reduction ~45% (43.49%) from 28.51MB to 16.11MB.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 156

• 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 100.0%

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Totals
Change from base Build 152:	0.0%
Covered Lines:	408
Relevant Lines:	408

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💛 - Coveralls

[stephen-soltesz]

Hello, @aorfanos -- thank you for your contributions! I have a few clarifying questions and small requests.

Reviewable status: 0 of 1 LGTMs obtained

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Dockerfile, line 8 at r1 (raw file):

ADD . ./

# TODO(soltesz): Use vgo for dependencies.

If you're making some edits, please remove this TODO. It predates go mod.

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Dockerfile, line 12 at r1 (raw file):

-w -s -extldflags '-static'

Can you say why these are necessary now?

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Dockerfile, line 15 at r1 (raw file):

    ./cmd/github_receiver

FROM gcr.io/distroless/static

Please add this link as a comment above FROM.

# See also: https://github.com/GoogleContainerTools/distroless/blob/main/base/README.md

I found this to confirm that the TLS certificates were part of this "distroless" image. Initially I briefly confused with this and FROM scratch.

[aorfanos]

hey @stephen-soltesz , happy to contribute to such an amazing project :)

Regarding this:

-w -s -extldflags '-static'

The flags -w -s are used to decrease image size by not including debugging information and the symbol table on the created executable.
Flag extldflags static use was to create a static binary including libraries at build time, but after checking it does not contribute to the executable size reduction I have removed it.

[stephen-soltesz]

Thank you!

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 LGTMs obtained