descriptor: Require RFC-6838-compliant mediaTypes

Previously we didn't define what a media type was and the old regular
expression (from 818a56a, schema: image manifest, 2016-04-12, opencontainers#18)
was stricter than the RFC.  The RFC rules are in [1], and the JSON
Schema regexp syntax is in [2].  In the new pattern,

* [A-Za-z0-9] is restricted-name-first (ALPHA / DIGIT), with ALPHA and
  DIGIT defined in RFC 5234 [3].
* [A-Za-z0-9!#$&-^_.+] is restricted-name-chars.

[1]: https://tools.ietf.org/html/rfc6838#section-4.2
[2]: https://tools.ietf.org/html/draft-wright-json-schema-validation-00#section-3.3
[3]: https://tools.ietf.org/html/rfc5234#appendix-B.1

Signed-off-by: W. Trevor King <[email protected]>

Removed the MUST for RFC 6838 from descriptor.md.

Signed-off-by: xiekeyang <[email protected]>

Author: wking

schema/defs-image.json

@@ -4,7 +4,7 @@
     "mediaType": {
       "id": "https://opencontainers.org/schema/image/mediaType",
       "type": "string",
-      "pattern": "^[a-z]+/[0-9a-zA-Z.+]+$"
+      "pattern": "^[A-Za-z0-9][A-Za-z0-9!#$&-^_.+]{0,126}/[A-Za-z0-9][A-Za-z0-9!#$&-^_.+]{0,126}$"
     },
     "digest": {
       "description": "the cryptographic checksum digest of the object, in the pattern '<hash>:<hexadecimal digest>'",