engine.#Pull implementation

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/dagger/engine/image.cue

@@ -0,0 +1,45 @@
+package engine
+
+// A ref is an address for a remote container image
+//
+// Examples:
+//   - "index.docker.io/dagger"
+//   - "dagger"
+//   - "index.docker.io/dagger:latest"
+//   - "index.docker.io/dagger:latest@sha256:a89cb097693dd354de598d279c304a1c73ee550fbfff6d9ee515568e0c749cfe"
+#Ref: string
+
+// Container image config
+// See https://opencontainers.org
+// https://github.com/moby/buildkit/blob/master/frontend/dockerfile/dockerfile2llb/image.go
+// https://github.com/opencontainers/image-spec/blob/main/specs-go/v1/config.go
+#ImageConfig: {
+	Env?: [...string]
+	User?: string
+	Cmd?: [...string]
+	...
+}
+
+// Download a container image from a remote repository
+#Pull: {
+	_type: "Pull"
+
+	// Repository source ref
+	source: #Ref
+
+	// Authentication
+	auth: [...{
+		target:   string
+		username: string
+		secret:   string | #Secret
+	}]
+
+	// Root filesystem of downloaded image
+	output: #FS
+
+	// Image digest
+	digest: string
+
+	// Downloaded container image config
+	config: #ImageConfig
+}