[Snyk] Security upgrade alpine from 3.4 to 3.13

[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, 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	Key Management Errors SNYK-ALPINE34-OPENSSL-374005	No Known Exploit
	400	Key Management Errors SNYK-ALPINE34-OPENSSL-374005	No Known Exploit
	300	Use of a Broken or Risky Cryptographic Algorithm SNYK-ALPINE34-OPENSSL-374280	No Known Exploit
	300	Information Exposure SNYK-ALPINE34-OPENSSL-374450	No Known Exploit
	300	CVE-2018-0733 SNYK-ALPINE34-OPENSSL-374732	No Known Exploit

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