Add docker.#Set for updating image metadata

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

pkg/universe.dagger.io/docker/test/docker.bats

@@ -22,3 +22,7 @@ setup() {
     dagger up ./run-export-directory-test.cue
     dagger up ./image-config-test.cue
 }
+
+@test "docker.#Set" {
+    dagger up ./set.cue
+}

pkg/universe.dagger.io/docker/test/set.cue

@@ -0,0 +1,40 @@
+package docker
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/dagger/engine"
+	"universe.dagger.io/docker"
+)
+
+dagger.#Plan & {
+	actions: {
+		image: output: docker.#Image & {
+			rootfs: engine.#Scratch
+			config: {
+				cmd: ["/bin/sh"]
+				env: PATH: "/sbin:/bin"
+				onbuild: ["COPY . /app"]
+			}
+		}
+		set: docker.#Set & {
+			input: image.output
+			config: {
+				env: FOO: "bar"
+				workdir: "/root"
+				onbuild: ["RUN /app/build.sh"]
+			}
+		}
+		verify: set.output.config & {
+			env: {
+				PATH: "/sbin:/bin"
+				FOO:  "bar"
+			}
+			cmd: ["/bin/sh"]
+			workdir: "/root"
+			onbuild: [
+				"COPY . /app",
+				"RUN /app/build.sh",
+			]
+		}
+	}
+}