Define URLs' scheme

[runcom]

Descriptor's props at https://github.com/opencontainers/image-spec/blob/master/descriptor.md#properties do not talk about which scheme a URL should have.
Should we define that URLs SHOULD have either http or https scheme? leaving other scheme (ftp, file) to be used as well, but enforcing http[s]? or should we leave it as is and maybe link https://tools.ietf.org/html/rfc1738?

/cc @vbatts @philips @jonboulle

[wking]

#431 is using JSON Schema's uri format, which requires RFC 3986. I think we should make that requirement explicit in the Markdown.

[vbatts]

interesting. Having it as URI is well known and supported by many libraries. Then having "a SHOULD support http/https" seems like a decent approach.

[wking]

On Thu, Nov 03, 2016 at 01:44:00PM -0700, Vincent Batts wrote:

Then having "a SHOULD support http/https" seems like a decent approach.

Sounds like a consensus :). I've filed #442 with suggested wording.