[Snyk] Security upgrade alpine from latest 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

• demo_app/Dockerfile

We recommend upgrading to alpine:3.15, as this image has only 0 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	Issue	Exploit Maturity
	CVE-2022-0778 SNYK-ALPINE315-OPENSSL-2426331	No Known Exploit
	CVE-2022-0778 SNYK-ALPINE315-OPENSSL-2426331	No Known Exploit

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