[Snyk] Security upgrade ruby from 2.5 to 2.6

[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:2.6, as this image has only 226 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
	571	Out-of-bounds Write SNYK-DEBIAN10-LIBDE265-2445048	No Known Exploit
	614	Use After Free SNYK-DEBIAN10-LIBXML2-2413976	No Known Exploit
	714	Buffer Overflow SNYK-DEBIAN10-OPENSSL-1569403	No Known Exploit
	614	Out-of-bounds Read SNYK-DEBIAN10-OPENSSL-1569406	No Known Exploit
	614	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN10-OPENSSL-2426310	No Known Exploit

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