[Snyk] Security upgrade alpine from 3.9 to 3.13.8

[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

• Dockerfile

We recommend upgrading to alpine:3.13.8, 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
	400	Improper Certificate Validation SNYK-ALPINE39-OPENSSL-1089232	No Known Exploit
	400	Improper Certificate Validation SNYK-ALPINE39-OPENSSL-1089232	No Known Exploit
	300	Integer Overflow or Wraparound SNYK-ALPINE39-OPENSSL-1089234	No Known Exploit
	400	Integer Overflow or Wraparound SNYK-ALPINE39-OPENSSL-1089235	No Known Exploit
	400	Integer Overflow or Wraparound SNYK-ALPINE39-OPENSSL-1089235	No Known Exploit

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