[Snyk] Security upgrade ruby from 2.7.4 to 2.7

[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 ruby:2.7, as this image has only 243 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
	786	Directory Traversal SNYK-DEBIAN11-DPKG-2847942	No Known Exploit
	614	Use After Free SNYK-DEBIAN11-LIBXML2-2413974	No Known Exploit
	614	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN11-OPENSSL-2426309	No Known Exploit
	714	OS Command Injection SNYK-DEBIAN11-OPENSSL-2807596	No Known Exploit
	614	Use After Free SNYK-DEBIAN11-SUBVERSION-2635643	No Known Exploit

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