[Snyk] Security upgrade alpine from 3.13.8 to 3.13.9

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• build/Dockerfile

We recommend upgrading to alpine:3.13.9, as this image has only 0 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	471	Out-of-bounds Write SNYK-ALPINE313-ZLIB-2434418	No Known Exploit

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[coveralls]

Pull Request Test Coverage Report for Build 2076106295

• 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 78.621%

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Totals
Change from base Build 2052918430:	0.0%
Covered Lines:	114
Relevant Lines:	145

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💛 - Coveralls