refactor(ecs): Asset ContainerImage no longer takes Construct arguments (#2906)

Turn the `ContainerImage` into the standard integration pattern. It is
no longer a construct itself, but creates subconstructs during `bind`.

BREAKING CHANGE:

* **ecs**: `ContainerImage.fromAsset()` now takes only build directory
  directly (no need to pass `scope` or `id` anymore).

Author: rix0rrr

packages/@aws-cdk/aws-codebuild/README.md

@@ -186,7 +186,7 @@ Alternatively, you can specify a custom image using one of the static methods on
   Hub.
 * Use `.fromEcrRepository(repo[, tag])` to reference an image available in an
   ECR repository.
-* Use `.fromAsset(this, id, { directory: dir })` to use an image created from a
+* Use `.fromAsset(directory)` to use an image created from a
   local asset.
 
 The following example shows how to define an image from a Docker asset: