image/spec: add missing "OnBuild" field

This field was added in commit 9f994c96468e2495a2dc118355b6565e7dac0f44,
which was merged before the image-spec v1.0.0 was released (which happened
in commit 79910625f0a7aa76590e4362817dda22f28343aa).

This patch backfills the specifications to describe the property.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

image/spec/v1.1.md

@@ -349,6 +349,16 @@ whitespace. It has been added to this example for clarity.
                 in the container. This value acts as a default and is replaced
                 by a working directory specified when creating a container.
             </dd>
+            <dt>
+                OnBuild <code>array of strings</code>
+            </dt>
+            <dd>
+                This metadata defines "trigger" instructions to be executed at
+                a later time, when the image is used as the base for another
+                build. Each trigger will be executed in the context of the
+                downstream build, as if it had been inserted immediately after
+                the *FROM* instruction in the downstream Dockerfile.
+            </dd>
         </dl>
     </dd>
     <dt>

image/spec/v1.2.md

@@ -405,6 +405,16 @@ whitespace. It has been added to this example for clarity.
                 in the container. This value acts as a default and is replaced
                 by a working directory specified when creating a container.
             </dd>
+            <dt>
+                OnBuild <code>array of strings</code>
+            </dt>
+            <dd>
+                This metadata defines "trigger" instructions to be executed at
+                a later time, when the image is used as the base for another
+                build. Each trigger will be executed in the context of the
+                downstream build, as if it had been inserted immediately after
+                the *FROM* instruction in the downstream Dockerfile.
+            </dd>
         </dl>
     </dd>
     <dt>

image/spec/v1.md

@@ -368,6 +368,16 @@ Here is an example image JSON file:
                 in the container. This value acts as a default and is replaced
                 by a working directory specified when creating a container.
             </dd>
+            <dt>
+                OnBuild <code>array of strings</code>
+            </dt>
+            <dd>
+                This metadata defines "trigger" instructions to be executed at
+                a later time, when the image is used as the base for another
+                build. Each trigger will be executed in the context of the
+                downstream build, as if it had been inserted immediately after
+                the *FROM* instruction in the downstream Dockerfile.
+            </dd>
         </dl>
     </dd>
 </dl>