Return to an Alpine-based Docker image

[bachya]

Describe what the PR does:

It turns out that #545 didn't really need anything from Debian, so let's return a smaller image.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.

[codecov]

Codecov Report

Merging #556 (12f4a08) into dev (b1fbe9f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev      #556   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           32        32           
  Lines         1707      1707           
=========================================
  Hits          1707      1707