Europa: use $dagger field to mark CUE types for engine recognition

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

stdlib/europa/dagger/engine/spec/engine/exec.cue

@@ -2,7 +2,7 @@ package engine
 
 // Execute a command in a container
 #Exec: {
-	_exec: {}
+	$dagger: task: _name: "Exec"
 
 	// Container filesystem
 	input: #FS

stdlib/europa/dagger/engine/spec/engine/fs.cue

@@ -2,19 +2,19 @@ package engine
 
 // A filesystem state
 #FS: {
-	_fs: ID: string
+	$dagger: fs: _id: string
 }
 
 // Produce an empty directory
 // FIXME: replace with a null value for #FS?
 #Scratch: {
-	_scratch: {}
+	$dagger: task: _name: "Scratch"
 
 	output: #FS
 }
 
 #ReadFile: {
-	_readFile: {}
+	$dagger: task: _name: "ReadFile"
 
 	input:    #FS
 	path:     string
@@ -23,7 +23,7 @@ package engine
 }
 
 #WriteFile: {
-	_writeFile: {}
+	$dagger: task: _name: "WriteFile"
 
 	input:    #FS
 	path:     string
@@ -33,7 +33,7 @@ package engine
 
 // Create a directory
 #Mkdir: {
-	_mkdir: {}
+	$dagger: task: _name: "Mkdir"
 
 	input: #FS
 
@@ -49,7 +49,7 @@ package engine
 }
 
 #Copy: {
-	_copy: {}
+	$dagger: task: _name: "Copy"
 
 	input: #FS
 	#CopyInfo
@@ -65,7 +65,7 @@ package engine
 }
 
 #Merge: {
-	_merge: {}
+	$dagger: task: _name: "Merge"
 
 	input: #FS
 	layers: [...#CopyInfo]

stdlib/europa/dagger/engine/spec/engine/git.cue

@@ -2,7 +2,7 @@ package engine
 
 // Push a directory to a git remote
 #GitPush: {
-	gitPush: {}
+	$dagger: task: _name: "GitPush"
 
 	input:  #FS
 	remote: string
@@ -11,7 +11,7 @@ package engine
 
 // Pull a directory from a git remote
 #GitPull: {
-	gitPull: {}
+	$dagger: task: _name: "GitPull"
 
 	remote: string
 	ref:    string

stdlib/europa/dagger/engine/spec/engine/image.cue

@@ -11,7 +11,7 @@ package engine
 
 // Upload a container image to a remote repository
 #Push: {
-	push: {}
+	$dagger: task: _name: "Push"
 
 	// Target repository address
 	dest: #Ref
@@ -35,7 +35,7 @@ package engine
 
 // Download a container image from a remote repository
 #Pull: {
-	pull: {}
+	$dagger: task: _name: "Pull"
 
 	// Repository source ref
 	source: #Ref
@@ -66,8 +66,9 @@ package engine
 #Ref: string
 
 // Build a container image using buildkit
+// FIXME: rename to #Dockerfile to clarify scope
 #Build: {
-	build: {}
+	$dagger: task: _name: "Build"
 
 	// Source directory to build
 	source: #FS

stdlib/europa/dagger/engine/spec/engine/secret.cue

@@ -2,5 +2,5 @@ package engine
 
 // An external secret
 #Secret: {
-	_secret: ID: string
+	$dagger: secret: _id: string
 }

stdlib/europa/dagger/engine/spec/engine/service.cue

@@ -1,6 +1,7 @@
 package engine
 
 // An external network service
+// FIXME: rename to endpoint?
 #Service: {
-	_service: ID: string
+	$dagger: service: _id: string
 }

stdlib/europa/dagger/engine/spec/engine/stream.cue

@@ -2,5 +2,5 @@ package engine
 
 // A stream of bytes
 #Stream: {
-	_stream: ID: string
+	$dagger: stream: _id: string
 }