First attempt at restoring the container.input interface

The goal is to preserve the container.input interface so that custom
images can be specified, as per yarn/test/test.cue while keeping the
initial fix for https://github.com/dagger/dagger/issues/1670

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

pkg/universe.dagger.io/yarn/test/test.cue

@@ -66,7 +66,7 @@ dagger.#Plan & {
 
 			build: yarn.#Build & {
 				source: common.data
-				container: input: buildImage.output
+				container: #input: buildImage.output
 			}
 		}
 	}