[Snyk] Security upgrade alpine from 3.12 to 3.13.7

[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.test

We recommend upgrading to alpine:3.13.7, 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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