[Snyk] Security upgrade alpine from latest to 3.15.4

[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:3.15.4, as this image has only 0 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

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