Update the auth spec scope grammar to reflect the fact that hostnames are optionally supported.

Signed-off-by: cyli <cyli@twistedmatrix.com>

docs/spec/auth/scope.md

@@ -56,7 +56,7 @@ it.
 The resource name represent the name which identifies a resource for a resource
 provider. A resource is identified by this name and the provided resource type.
 An example of a resource name would be the name component of an image tag, such
-as "samalba/myapp".
+as "samalba/myapp" or "hostname/samalba/myapp".
 
 ### Resource Actions
 
@@ -83,16 +83,24 @@ scopes.
 scope                   := resourcescope [ ' ' resourcescope ]*
 resourcescope           := resourcetype  ":" resourcename  ":" action [ ',' action ]*
 resourcetype            := /[a-z]*/
-resourcename            := component [ '/' component ]*
+resourcename            := [ hostname '/' ] component [ '/' component ]*
+hostname                := hostcomponent ['.' hostcomponent]* [':' port-number]
+hostcomponent           := /([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])/
+port-number             := /[0-9]+/
 action                  := /[a-z]*/
 component               := alpha-numeric [ separator alpha-numeric ]*
 alpha-numeric           := /[a-z0-9]+/
 separator               := /[_.]|__|[-]*/
 ```
 Full reference grammar is defined
-(here)[https://godoc.org/github.com/docker/distribution/reference]. Currently
-the scope name grammar is a subset of the reference grammar without support
-for hostnames.
+[here](https://godoc.org/github.com/docker/distribution/reference). Currently
+the scope name grammar is a subset of the reference grammar.
+
+> **NOTE:** that the `resourcename` may contain one `:` due to a possible port
+> number in the hostname component of the `resourcename`, so a naive
+> implementation that interprets the first three `:`-delimited tokens of a
+> `scope` to be the `resourcetype`, `resourcename`, and a list of `action`
+> would be insufficient.
 
 ## Resource Provider Use