Add documentation for building container images

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

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

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"universe.dagger.io/docker"
+)
+
+dagger.#Plan & {
+	client: filesystem: ".": read: contents: dagger.#FS
+
+	actions: build: docker.#Build & {
+		steps: [
+			docker.#Pull & {
+				source: "python:3.9"
+			},
+			docker.#Copy & {
+				contents: client.filesystem.".".read.contents
+				dest:     "/app"
+			},
+			docker.#Run & {
+				command: {
+					name: "pip"
+					args: ["install", "-r", "/app/requirements.txt"]
+				}
+			},
+			docker.#Set & {
+				config: cmd: ["python", "/app/app.py"]
+			},
+		]
+	}
+}