fix: handle missing healthcheck keys in config

[piksel]

Only perform the container.GetCreateConfig default value override dance for healthcheck when it's actually present in both container and image config.

Fixes #1809

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd54055) 70.61% compared to head (dee7d6a) 70.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1810      +/-   ##
==========================================
+ Coverage   70.61%   70.63%   +0.01%     
==========================================
  Files          26       26              
  Lines        2488     2489       +1     
==========================================
+ Hits         1757     1758       +1     
  Misses        631      631              
  Partials      100      100              

Files	Coverage Δ
pkg/container/container.go	71.15% <100.00%> (+0.13%)	⬆️

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