rename engine.#Build to engine.#Dockerfile

Signed-off-by: Richard Jones <richard@dagger.io>

tests/tasks/dockerfile/dockerfile.cue

@@ -0,0 +1,20 @@
+package testing
+
+import (
+	"dagger.io/dagger/engine"
+)
+
+engine.#Plan & {
+	inputs: directories: testdata: path: "./testdata"
+
+	actions: {
+		build: engine.#Dockerfile & {
+			source: inputs.directories.testdata.contents
+		}
+
+		verify: engine.#Exec & {
+			input: build.output
+			args: ["sh", "-c", "test $(cat /dir/foo) = foobar"]
+		}
+	}
+}