Do not assume Linux OCI images (can be useful for FreeBSD, etc.)

[nohajc]

I wanted to unpack a FreeBSD OCI image and had to remove the PlatformOS check first. Would it be OK to drop it altogether?

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.07%. Comparing base (a058839) to head (4d06d32).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
+ Coverage   72.89%   74.07%   +1.18%     
==========================================
  Files          71       71              
  Lines        6164     4633    -1531     
==========================================
- Hits         4493     3432    -1061     
+ Misses       1213      748     -465     
+ Partials      458      453       -5     

Files with missing lines	Coverage Δ
oci/config/convert/runtime.go	77.55% <ø> (+5.24%)	⬆️

... and 59 files with indirect coverage changes

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[cyphar]

You can't just remove the check because that code generates an OCI runtime-spec config.json and it currently can only generate Linux-specific ones. We just merged FreeBSD support into the spec, so if you'd like FreeBSD support I would suggest the following course of action:

1. Split the logic into per-OS files with //go:build.
2. Add code to generate FreeBSD config.jsons.
3. Figure out how to run the CI tests on FreeBSD, and fix any issues that come up.

[nohajc]

I see. Thanks for the explanation. I only needed to unpack an image at this point. Can't say if I'll find time to work on this properly, so feel free to close the PR.

[cyphar]

If you just want to unpack an image, umoci raw unpack should work -- though this won't work with umoci repack. Maybe we need a flag for umoci unpack to disable generating a config.json...

[nohajc]

Thank you for the tip. Haven't realized there's another version of the unpack command. That should work for me.