Add documentation for building container images

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

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

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"universe.dagger.io/docker"
+)
+
+dagger.#Plan & {
+	actions: versions: {
+		"8.0": _
+		"5.7": _
+
+		// This is a template
+		// See https://cuelang.org/docs/tutorials/tour/types/templates/
+		[tag=string]: {
+			build: docker.#Build & {
+				steps: [
+					docker.#Pull & {
+						source: "mysql:\(tag)"
+					},
+					docker.#Set & {
+						config: cmd: [
+							"--character-set-server=utf8mb4",
+							"--collation-server=utf8mb4_unicode_ci",
+						]
+					},
+				]
+			}
+			push: docker.#Push & {
+				image: build.output
+				dest:  "registry.example.com/mysql:\(tag)"
+			}
+		}
+	}
+}