rename engine.#Build to engine.#Dockerfile

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

tests/tasks/dockerfile/build_args.cue

@@ -0,0 +1,19 @@
+package testing
+
+import (
+	"dagger.io/dagger/engine"
+)
+
+engine.#Plan & {
+	inputs: directories: testdata: path: "./testdata"
+
+	actions: build: engine.#Dockerfile & {
+		source: inputs.directories.testdata.contents
+		dockerfile: contents: """
+			FROM alpine:latest@sha256:ab00606a42621fb68f2ed6ad3c88be54397f981a7b70a79db3d1172b11c4367d
+			ARG TEST=foo
+			RUN test "${TEST}" = "bar"
+			"""
+		buildArg: TEST: "bar"
+	}
+}