[docs] Add description of "X-Registry-Config"

The structure of this header has never been specified in the docs and has
been a pain point for those who have tried to get registry authentication
to work during builds. This Patch *finally* adds a description of JSON
object which should be base64-url-safe-encoded and used as the value of
the X-Registry-Config HTTP request header during `POST /build`.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

docs/reference/api/docker_remote_api_v1.20.md

@@ -1341,7 +1341,28 @@ Query Parameters:
     Request Headers:
 
 -   **Content-type** – Set to `"application/tar"`.
--   **X-Registry-Config** – base64-encoded ConfigFile object
+-   **X-Registry-Config** – A base64-url-safe-encoded Registry Auth Config JSON
+        object with the following structure:
+
+            {
+                "docker.example.com": {
+                    "username": "janedoe",
+                    "password": "hunter2"
+                },
+                "https://index.docker.io/v1/": {
+                    "username": "mobydock",
+                    "password": "conta1n3rize14"
+                }
+            }
+
+        This object maps the hostname of a registry to an object containing the
+        "username" and "password" for that registry. Multiple registries may
+        be specified as the build may be based on an image requiring
+        authentication to pull from any arbitrary registry. Only the registry
+        domain name (and port if not the default "443") are required. However
+        (for legacy reasons) the "official" Docker, Inc. hosted registry must
+        be specified with both a "https://" prefix and a "/v1/" suffix even
+        though Docker will prefer to use the v2 registry API.
 
 Status Codes:
 

docs/reference/api/docker_remote_api_v1.21.md

@@ -1351,7 +1351,28 @@ Query Parameters:
     Request Headers:
 
 -   **Content-type** – Set to `"application/tar"`.
--   **X-Registry-Config** – base64-encoded ConfigFile object
+-   **X-Registry-Config** – A base64-url-safe-encoded Registry Auth Config JSON
+        object with the following structure:
+
+            {
+                "docker.example.com": {
+                    "username": "janedoe",
+                    "password": "hunter2"
+                },
+                "https://index.docker.io/v1/": {
+                    "username": "mobydock",
+                    "password": "conta1n3rize14"
+                }
+            }
+
+        This object maps the hostname of a registry to an object containing the
+        "username" and "password" for that registry. Multiple registries may
+        be specified as the build may be based on an image requiring
+        authentication to pull from any arbitrary registry. Only the registry
+        domain name (and port if not the default "443") are required. However
+        (for legacy reasons) the "official" Docker, Inc. hosted registry must
+        be specified with both a "https://" prefix and a "/v1/" suffix even
+        though Docker will prefer to use the v2 registry API.
 
 Status Codes: