docker→OCI conversion should differentiate compressed and uncompressed layers

[mtrmac]

Right now, in manifestSchema2.convertToManifestOCI1, we assume that all input layers are gzipped, which is not generally the case. We should somehow detect this (e.g. copy.copyLayers does actually know the true format at some point) and use the correct MIME type.

[runcom]

I also noticed we should fix the image config when converting from docker v2s2 to OCI 😢 right now we're blindly just changing docker config media type to OCI config media type but that's wrong because docker v2s2 config has "container_config" which OCI doesn't have. Let's merge #223 and re-iterate ok?

[mtrmac]

Yes, that’s why I filed this as a separate issue.

[runcom]

Right now, in manifestSchema2.convertToManifestOCI1, we assume that all input layers are gzipped,

schema2 to oci1 is always gzipped layers afaict - there's no such "no-gzipped" layers in docker v2s2

[mtrmac]

Right now, in manifestSchema2.convertToManifestOCI1, we assume that all input layers are gzipped,

schema2 to oci1 is always gzipped layers afaict - there's no such "no-gzipped" layers in docker v2s2

Good point, dockerd always compresses the layers when pushing; but it does do format autodetection when pulling and supports both uncompressed layers and non-gzip compression formats.

When both us and dockerd send layers compressed using gzip, so just assuming gzip should be good enough.