[Snyk] Security upgrade alpine from 3.10 to 3.11.12

[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

• tools/docker/curl_image/Dockerfile

We recommend upgrading to alpine:3.11.12, 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 Read SNYK-ALPINE310-APKTOOLS-1534688	No Known Exploit

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[pixie-io-buildbot]

Can one of the admins verify this patch?

[vihangm]

Obsolete see #355