chore(backport): Apply Docker build check corrections

[meeseeksmachine]

Backport PR #2543: chore: Apply Docker build check corrections

* Backport PR https://github.com/scikit-hep/pyhf/pull/2543
* Apply revisions to Dockerfile from Docker build checks.
   - https://docs.docker.com/reference/build-checks/from-as-casing/
   - https://docs.docker.com/go/dockerfile/rule/undefined-var/
   - https://docs.docker.com/go/dockerfile/rule/legacy-key-value-format/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2544   +/-   ##
=================================================
  Coverage                  ?   98.19%           
=================================================
  Files                     ?       69           
  Lines                     ?     4547           
  Branches                  ?      804           
=================================================
  Hits                      ?     4465           
  Misses                    ?       49           
  Partials                  ?       33           

Flag	Coverage Δ
contrib	97.73% <ø> (?)
doctest	98.02% <ø> (?)
unittests-3.10	96.19% <ø> (?)
unittests-3.11	96.19% <ø> (?)
unittests-3.12	96.19% <ø> (?)
unittests-3.7	96.17% <ø> (?)
unittests-3.8	96.21% <ø> (?)
unittests-3.9	96.23% <ø> (?)

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