fix: use default http transport for head

[piksel]

Restore default http transport settings, including proxy auto-detection for HEAD requests.

Fixes #922

[codecov]

Codecov Report

Merging #926 (98e5199) into main (b4cf17d) will increase coverage by 6.21%.
The diff coverage is 15.38%.

@@            Coverage Diff             @@
##             main     #926      +/-   ##
==========================================
+ Coverage   40.14%   46.35%   +6.21%     
==========================================
  Files          25       25              
  Lines        1415     1426      +11     
==========================================
+ Hits          568      661      +93     
+ Misses        785      697      -88     
- Partials       62       68       +6     

Impacted Files	Coverage Δ
pkg/registry/digest/digest.go	0.00% <0.00%> (ø)
pkg/notifications/shoutrrr.go	73.43% <100.00%> (+0.42%)	⬆️
pkg/notifications/email.go	90.90% <0.00%> (+4.54%)	⬆️
pkg/container/client.go	18.45% <0.00%> (+18.45%)	⬆️
pkg/container/container.go	36.70% <0.00%> (+36.70%)	⬆️
pkg/container/metadata.go	63.63% <0.00%> (+63.63%)	⬆️
pkg/container/util.go	100.00% <0.00%> (+100.00%)	⬆️

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