[Snyk] Security upgrade alpine from 3.14 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

• scripts/docker/letsencrypt/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
	221	CVE-2022-37434 SNYK-ALPINE314-ZLIB-2976174	No Known Exploit

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[codecov[bot]]

Codecov Report

Merging #9825 (9b56ca2) into master (afc9194) will decrease coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9825      +/-   ##
==========================================
- Coverage   61.33%   61.32%   -0.01%     
==========================================
  Files         822      822              
  Lines       50277    50277              
  Branches     7748     7748              
==========================================
- Hits        30837    30832       -5     
- Misses      17760    17763       +3     
- Partials     1680     1682       +2