docker.#Build: support for nested builds

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

pkg/universe.dagger.io/docker/test/nested-build-test.cue

@@ -0,0 +1,26 @@
+package docker
+
+import (
+	"dagger.io/dagger"
+	"universe.dagger.io/docker"
+)
+
+dagger.#Plan & {
+	actions: build: docker.#Build & {
+		steps: [
+			docker.#Build & {
+				steps: [
+					docker.#Pull & {
+						source: "alpine"
+					},
+					docker.#Run & {
+						cmd: name: "ls"
+					},
+				]
+			},
+			docker.#Run & {
+				cmd: name: "ls"
+			},
+		]
+	}
+}