[Snyk] Security upgrade ruby from 2.6.5-slim to 2.7.8-slim

[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:2.7.8-slim, as this image has only 46 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
	714	Directory Traversal SNYK-DEBIAN10-DPKG-2847944	No Known Exploit
	714	Out-of-bounds Read SNYK-DEBIAN10-LIBTASN16-3061094	No Known Exploit
	714	Buffer Overflow SNYK-DEBIAN10-OPENSSL-1569403	No Known Exploit
	714	OS Command Injection SNYK-DEBIAN10-OPENSSL-2807585	No Known Exploit
	714	OS Command Injection SNYK-DEBIAN10-OPENSSL-2933515	No Known Exploit

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🦉 Directory Traversal