Docs: add content to 1205: "building container images"

Signed-off-by: Solomon Hykes <solomon@dagger.io>

docs/tests/core-concepts/container-images/simple/build.cue

@@ -5,16 +5,23 @@ import (
 	"universe.dagger.io/docker"
 )
 
-dagger.#Plan & {
-	client: filesystem: "./src": read: contents: dagger.#FS
+// This action builds a docker image from a python app.
+// Build steps are defined in native CUE.
+#PythonBuild: {
+	// Source code of the Python application
+	app: dagger.#FS
 
-	actions: build: docker.#Build & {
+	// Container image
+	image: _build.output
+
+	// Build steps
+	_build: docker.#Build & {
 		steps: [
 			docker.#Pull & {
 				source: "python:3.9"
 			},
 			docker.#Copy & {
-				contents: client.filesystem."./src".read.contents
+				contents: app
 				dest:     "/app"
 			},
 			docker.#Run & {
@@ -29,3 +36,12 @@ dagger.#Plan & {
 		]
 	}
 }
+
+// Example usage in a plan
+dagger.#Plan & {
+	client: filesystem: "./src": read: contents: dagger.#FS
+
+	actions: build: #PythonBuild & {
+		app: client.filesystem."./src".read.contents
+	}
+}