image/spec: add missing "Shell" field (v1.2)

This field was added in b18ae8c9ccc2eb6cf8aa947f25eb6f1d20089776, which
was part of v1.12.0-rc1 and later, which used image spec v1.2.0.

This patch amends the v1.2 spec to include the missing field.

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

image/spec/v1.2.md

@@ -427,6 +427,17 @@ whitespace. It has been added to this example for clarity.
                 downstream build, as if it had been inserted immediately after
                 the *FROM* instruction in the downstream Dockerfile.
             </dd>
+            <dt>
+                Shell <code>array of strings</code>
+            </dt>
+            <dd>
+                Override the default shell used for the *shell* form of
+                commands during "build". The default shell on Linux is
+                <code>["/bin/sh", "-c"]</code>, and <code>["cmd", "/S", "/C"]</code>
+                on Windows. This field is set by the <code>SHELL</code>
+                instruction in a Dockerfile, and *must* be written in JSON
+                form.
+            </dd>
         </dl>
     </dd>
     <dt>