Show types during variable list operation

If a type was explicitly provided, it will be displayed in the variable
listing.  Inferred type names are not displayed, as they likely would
not match the user's intent.

Previously only `string` and `bool` default values were displayed in the
 listing.  All default values, regardless of type, are now displayed.

Signed-off-by: Roberto Villarreal <rrjjvv@yahoo.com>

docs/reference/buildx_bake.md

@@ -198,12 +198,15 @@ To list variables:
 
 ```console
 $ docker buildx bake --list=variables
-VARIABLE      VALUE                DESCRIPTION
-REGISTRY      docker.io/username   Registry and namespace
-IMAGE_NAME    my-app               Image name
-GO_VERSION    <null>
+VARIABLE      TYPE      VALUE                DESCRIPTION
+REGISTRY      string    docker.io/username   Registry and namespace
+IMAGE_NAME    string    my-app               Image name
+GO_VERSION              <null>
+DEBUG         bool      false                Add debug symbols
 ```
 
+Variable types will be shown when set using the `type` property in the Bake file.
+
 By default, the output of `docker buildx bake --list` is presented in a table
 format. Alternatively, you can use a long-form CSV syntax and specify a
 `format` attribute to output the list in JSON.