Add missing volume claim to user-container example

[jeongukjae]

Pull Request Checklist

• Fixes Example "user_container.py" doesn't work #973
• Tests added not needed here
• Documentation/examples added not needed here
• Good commit messages and/or PR title

Description of PR
Following the volumes page in argo workflows document, updated user-container examples to have volumeClaimTemplates.

• https://argo-workflows.readthedocs.io/en/latest/walk-through/volumes/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.9%. Comparing base (3266a5b) to head (186b5e7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1242   +/-   ##
=====================================
  Coverage   45.9%   45.9%           
=====================================
  Files         60      60           
  Lines       4082    4082           
  Branches     857     857           
=====================================
  Hits        1875    1875           
  Misses      2175    2175           
  Partials      32      32           

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