Europa: docker: split in smaller files

Signed-off-by: Solomon Hykes <solomon@dagger.io>

europa-universe/docker/pull.cue

@@ -0,0 +1,37 @@
+// Build, ship and run Docker containers in Dagger
+package docker
+
+import (
+	"dagger.io/dagger/engine"
+	"dagger.io/dagger"
+)
+
+// Download an image from a remote registry
+#Pull: {
+	// Source ref.
+	source: #Ref
+
+	// Registry authentication
+	// Key must be registry address, for example "index.docker.io"
+	auth: [registry=string]: {
+		username: string
+		secret:   dagger.#Secret
+	}
+
+	_op: engine.#Pull & {
+		"source": source
+		"auth": [ for target, creds in auth {
+			"target": target
+			creds
+		}]
+	}
+
+	// Downloaded image
+	image: #Image & {
+		rootfs: _op.output
+		config: _op.config
+	}
+
+	// FIXME: compat with Build API
+	output: image
+}