Add documentation for building container images

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

docs/tests/core-concepts/container-images/plans/with-dockerfile.cue

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"dagger.io/dagger"
+)
+
+dagger.#Plan & {
+	client: filesystem: ".": read: contents: dagger.#FS
+
+	actions: build: dagger.#Dockerfile & {
+		// This is the context.
+		source: client.filesystem.".".read.contents
+
+		// Default is to look for a Dockerfile in the context,
+		// but let's declare it here.
+		contents: #"""
+			FROM python:3.9
+			COPY . /app
+			RUN pip install -r /app/requirements.txt
+			CMD python /app/app.py
+
+			"""#
+	}
+}