fix(docker): target the dev stage when building for local development

docker-compose.override.yml only overrode the dockerfile, so it kept
inheriting target: final from docker-compose.yml — but Dockerfile.dev
is single-stage and has no final stage, so the dev build failed. Name
the stage in Dockerfile.dev and target it explicitly from the
override.

docker-compose.override.yml

@@ -4,6 +4,7 @@ services:
   graph:
     build:
       dockerfile: Dockerfile.dev
+      target: dev
     volumes:
       - .:/app
       - ./vendor:/app/vendor