Add a devcontainer on alpine

[MichaelChirico]

This is for #6350.

After the investigation, I am OK kicking the can on this one a bit further. I wanted to investigate since on first glance, forder() being length-0 looked more concerning. But in actuality, it just means "input is sorted", which is the same as base::order() finds, hence not show-stopping -- this is pretty deeply edge-case behavior anyway.

I don't know enough about how forder() handles strings (especially w.r.t. encoding); AFAICT this will be a lot easier to debug after #6669 (since we can then use print-debugging to see how different parts of the routine give different results on different platforms).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (70c64ac) to head (8c20925).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6710   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          79       79           
  Lines       14553    14553           
=======================================
  Hits        14352    14352           
  Misses        201      201           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ben-schwen]

LGTM