validate.go: add the verification content

[zhouhao3]

According to this modified.
Signed-off-by: zhouhao zhouhao@cn.fujitsu.com

[zhouhao3]

Here I have a doubt that there is no reason for the validation of the mediatype field, because the mediatype field is optional and not always present in the descriptor.

[wking]

48fcec2 looks good to me.

[stevvooe]

It might be better to combine this into a function that tells whether or not a mediatype is a layer, although, this validation may be too strict.

[zhouhao3]

It might be better to combine this into a function that tells whether or not a mediatype is a layer, although, this validation may be too strict.

@stevvooe The judgment here is only a warning, not an error. So it can not be said that the conditions are not layers.So I do not think your idea is good.

[vbatts]

LGTM

[cyphar]

@wking As @q384566678 said, this is only a warning -- the discussion inside opencontainers/image-tools#111 is not completely valid (this schema validator is not meant to error on unknown layer types).

[zhouhao3]

@philips @jonboulle @stevvooe PTAL

[jonboulle]

lgtm

[stevvooe]

As @cyphar said, this is probably too aggressive.