[Snyk] Security upgrade alpine from 3.7 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

• contrib/docker/cookiecutter-with-jinja2-extensions/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	Priority Score / 1000	Issue	Exploit Maturity
	500	Out-of-bounds Write SNYK-ALPINE37-MUSL-458286	No Known Exploit
	500	Out-of-bounds Write SNYK-ALPINE37-MUSL-458286	No Known Exploit

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