[Snyk] Security upgrade alpine from 3.15.4 to 3.15

[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:3.15, as this image has only 4 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
	300	Inadequate Encryption Strength SNYK-ALPINE315-OPENSSL-2941810	No Known Exploit
	300	Inadequate Encryption Strength SNYK-ALPINE315-OPENSSL-2941810	No Known Exploit
	221	CVE-2022-4304 SNYK-ALPINE315-OPENSSL-3314628	No Known Exploit
	221	CVE-2022-4450 SNYK-ALPINE315-OPENSSL-3314629	No Known Exploit
	500	Out-of-bounds Write SNYK-ALPINE315-ZLIB-2976173	No Known Exploit

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🦉 Inadequate Encryption Strength