mutate: let Time and Canonical take tarball.LayerOption

[mzihlmann]

Time rebuilds the image onto empty.Image and re-tars every layer, so every layer gets compressed again. layerTime always gzips and calls tarball.LayerFromOpener with no options, so a rebuilt layer is always a types.DockerLayer. An OCI image comes back as docker schema2 and a zstd layer is silently downgraded to gzip. Canonical inherits both.

This threads tarball.LayerOption through to LayerFromOpener and hands the opener the uncompressed tar instead of pre-gzipping it. The layer is then compressed once, in the codec the caller asked for. Pair it with the existing mutate.MediaType and mutate.ConfigMediaType to bring the manifest and config along.

The options are variadic so existing call sites are unaffected, and with none passed the output is byte identical to before. I checked that across docker and oci, gzip and zstd, uncompressed, foreign and multi layer images. One incompatibility remains, Time and Canonical can no longer be assigned to a func(v1.Image) (v1.Image, error) variable.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.42%. Comparing base (7b32099) to head (67a976b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2403   +/-   ##
=======================================
  Coverage   57.42%   57.42%           
=======================================
  Files         166      166           
  Lines       11558    11558           
=======================================
  Hits         6637     6637           
  Misses       4142     4142           
  Partials      779      779           

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