[Snyk] Security upgrade nginx from 1.15.5 to 1.20

[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 nginx:1.20, as this image has only 72 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
	364	Memory Leak SNYK-DEBIAN9-LIBXML2-539774	No Known Exploit
	364	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN9-LIBXML2-542924	No Known Exploit
	364	Memory Leak SNYK-DEBIAN9-LIBXML2-542930	No Known Exploit
	364	Out-of-bounds Read SNYK-DEBIAN9-LIBXML2-609786	No Known Exploit
	364	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN9-TAR-312293	No Known Exploit

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