Fix case of 'AS' in Dockerfile

[dhirving]

GitHub is complaining about this on every PR review now.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.53%. Comparing base (05928f5) to head (d42cd29).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1058   +/-   ##
=======================================
  Coverage   89.53%   89.53%           
=======================================
  Files         361      361           
  Lines       46200    46200           
  Branches     9469     9469           
=======================================
  Hits        41366    41366           
  Misses       3499     3499           
  Partials     1335     1335           

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