change ssh property to a simple collection (list or dict)

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

build.md

@@ -157,7 +157,7 @@ args:
 
 ### ssh
 
-`ssh` defines an SSH authentication that the image builder SHOULD use during image build (e.g., cloning private repository)
+`ssh` defines SSH authentications that the image builder SHOULD use during image build (e.g., cloning private repository)
 
 `ssh` property syntax can be either:
 * `default` - let the builder connect to the ssh-agent.
@@ -167,13 +167,15 @@ Simple`default` sample
 ```yaml
 build:
   context: .
-  ssh: default   # mount the default ssh agent
+  ssh: 
+    default   # mount the default ssh agent
 ```
 Using a custom id `myproject` with path to a local SSH key:
 ```yaml
 build:
   context: .
-  ssh: myproject=~/.ssh/myproject.pem
+  ssh: 
+    - myproject=~/.ssh/myproject.pem
 ```
 Image builder can then rely on this to mount SSH key during build.
 For illustration, [Buildkit extended syntax](https://github.com/compose-spec/compose-spec/pull/234/%5Bmoby/buildkit@master/frontend/dockerfile/docs/syntax.md#run---mounttypessh%5D(https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md#run---mounttypessh)) can be used to mount ssh key set by ID and access a secured resource:

schema/compose-spec.json

@@ -91,12 +91,7 @@
                 "context": {"type": "string"},
                 "dockerfile": {"type": "string"},
                 "args": {"$ref": "#/definitions/list_or_dict"},
-                "ssh": {
-                  "oneOf": [
-                    {"type": "string"},
-                    {"$ref": "#/definitions/list_or_dict"}
-                  ]
-                },
+                "ssh": {"$ref": "#/definitions/list_or_dict"},
                 "labels": {"$ref": "#/definitions/list_or_dict"},
                 "cache_from": {"type": "array", "items": {"type": "string"}},
                 "cache_to": {"type": "array", "items": {"type": "string"}},