nydusify: fix unnecessary manifest index when copy one platform image

[imeoer]

When use the command to copy the image with specified one platform:

nydusify copy --platform linux/amd64 --source nginx --target localhost:5000/nginx

We found the target image is a manifest index format like:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
  "manifests": [
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "digest": "sha256:d2e65182b5fd330470eca9b8e23e8a1a0d87cc9b820eb1fb3f034bf8248d37ee",
      "size": 1778,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    }
  ]
}

This can be a bit strange, in fact just the manifest is enough, the patch improves this.

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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• I have updated the documentation accordingly.
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[codecov]

Codecov Report

Merging #1479 (6a34896) into master (c9fbce8) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1479      +/-   ##
==========================================
- Coverage   62.50%   62.49%   -0.02%     
==========================================
  Files         123      123              
  Lines       43148    43148              
  Branches    43148    43148              
==========================================
- Hits        26971    26966       -5     
- Misses      14870    14874       +4     
- Partials     1307     1308       +1     

see 2 files with indirect coverage changes

[adamqqqplay]

Thanks for your fix!