Update docker/dockerfile Docker tag to v1.21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
docker/dockerfile	syntax	minor	1.20 → 1.21

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.50%. Comparing base (9e1e007) to head (f8bcf92).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   43.50%   43.50%           
=======================================
  Files          73       73           
  Lines        8422     8422           
=======================================
  Hits         3664     3664           
  Misses       4758     4758           

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