Ensure we can Marshall #Scratch as CUE

Signed-off-by: Joel Longtine <joel@dagger.io>

tests/tasks.bats

@@ -92,6 +92,8 @@ setup() {
 @test "task: #Scratch" {
     cd "$TESTDIR"/tasks/scratch
     "$DAGGER" --europa up ./scratch.cue -l debug
+    "$DAGGER" --europa up ./scratch_build_scratch.cue -l debug
+    "$DAGGER" --europa up ./scratch_writefile.cue -l debug
 }
 
 @test "task: #Subdir" {

tests/tasks/scratch/scratch_build_scratch.cue

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+engine.#Plan & {
+	actions: {
+		write: engine.#WriteFile & {
+			input:       engine.#Scratch
+			path:        "/.dockerignore"
+			contents:    "Dockerfile"
+			permissions: 700
+		}
+
+		build: engine.#Build & {
+			source: write.output
+			dockerfile: contents: """
+				FROM scratch
+				"""
+			// Assert that output is engine.#Scratch
+			output: engine.#Scratch
+		}
+	}
+}

tests/tasks/scratch/scratch_writefile.cue

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+engine.#Plan & {
+	actions: {
+		write: engine.#WriteFile & {
+			input:       engine.#Scratch
+			path:        "/testing"
+			contents:    "1,2,3"
+			permissions: 700
+		}
+		readfile: engine.#ReadFile & {
+			input: write.output
+			path:  "/testing"
+		} & {
+			// assert result
+			contents: "1,2,3"
+		}
+	}
+}