sparse: do not modify media type when unspecified

[jabrown85]

The media type should only be modified when the user specifies it. In my case I noticed the digest given by the sparse.Image was not the same as the image it was dervied from. This was because the logic to set the media type defaulted to overriding to OCI if the media type was not specified. This commit changes the logic to only override the media type if the user specifies it.

[jabrown85]

I don't think lifecycle is aware of sparse vs layout at all today and this should change nothing there.

[jjbustamante]

Are we going to need to change the lifecycle at all? Or should that be covered by "has base image"?

As I mentioned on Slack:

• Originally I was always overriding the media types to use OCI because tooling like umoci, which I used to verify the image was working, didn't recognize docker media types.
• I would like to add a flag similar to skopeo copy to allow users to override the media type if they want to:

  -f, --format string                         MANIFEST TYPE (oci, v2s1, or v2s2) to use in the destination (default is manifest type of source, with fallbacks)

  • In this case, I think we will need to make some changes in lifecycle and pack to pass through the media-type option, but I think this can just be an improvement.