Merge pull request #133 from dagger/go-no-copy

stdlib: go: mount source rather than copying

stdlib/go/go.cue

@@ -21,12 +21,7 @@ import (
 		dagger.#FetchContainer & {
 			ref: "docker.io/golang:\(version)-alpine"
 		},
-		dagger.#Copy & {
-			from: source
-			dest: "/src"
-		},
 		dagger.#Exec & {
-			dir:    "/src"
 			"args": ["go"] + args
 
 			env: env
@@ -38,6 +33,9 @@ import (
 				GOPATH: "/go"
 			}
 
+			dir: "/src"
+			mount: "/src": from: source
+
 			mount: "/root/.cache": "cache"
 		},
 	]