fix: missing digests on manifests

[caarlos0]

it was missing the digest parsing for the manifest, and there were no tests covering it.

now the tests are there, and so is the fix.

refs #3599

[codecov]

Codecov Report

Merging #3602 (274b9b3) into main (f05f3b5) will decrease coverage by 0.04%.
The diff coverage is 55.00%.

@@            Coverage Diff             @@
##             main    #3602      +/-   ##
==========================================
- Coverage   83.88%   83.83%   -0.05%     
==========================================
  Files         117      117              
  Lines        9790     9798       +8     
==========================================
+ Hits         8212     8214       +2     
- Misses       1271     1275       +4     
- Partials      307      309       +2     

Impacted Files	Coverage Δ
internal/pipe/docker/api.go	100.00% <ø> (ø)
internal/pipe/docker/api_docker.go	83.63% <53.84%> (-4.60%)	⬇️
internal/pipe/docker/manifest.go	94.11% <57.14%> (-2.83%)	⬇️

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