Fix bugs in Dart Dockerfile and add comments

dart_grpc_bench/Dockerfile

@@ -1,28 +1,36 @@
-FROM dart:3.3.4 AS build
-
-WORKDIR /app
-COPY dart_grpc_bench/pubspec.yaml /app/pubspec.yaml
-COPY proto /app/proto
-
-RUN dart pub get
-COPY dart_grpc_bench /app
-# Ensure packages are still up-to-date if anything has changed
-RUN dart pub get --offline
-RUN apt update && apt install -y protobuf-compiler
-RUN dart pub global activate protoc_plugin
-RUN mkdir -p lib/src/generated
-RUN protoc --plugin=protoc-gen-dart=$HOME/.pub-cache/bin/protoc-gen-dart --proto_path=/app/proto/helloworld --dart_out=grpc:lib/src/generated -Iproto /app/proto/helloworld/helloworld.proto
-
-RUN dart compile exe bin/server.dart -o bin/server
-
-# ENTRYPOINT [ "/usr/bin/dart", "/app/bin/server.dart" ]
-
-# Build minimal serving image from AOT-compiled `/server` and required system
-# libraries and configuration files stored in `/runtime/` from the build stage.
-FROM scratch
-COPY --from=build /runtime/ /
-COPY --from=build /app/bin/server /app/bin/
-
-# Start server.
-EXPOSE 50051
+FROM dart:3.3.4 AS build
+
+# Resolve app dependencies.
+WORKDIR /app
+COPY pubspec.* ./
+COPY proto ./proto
+RUN dart pub get
+
+# Copy app source code
+COPY . .
+
+# Ensure packages are still up-to-date if anything has changed
+RUN dart pub get --offline
+
+# Setup protobuf compiler
+RUN apt update && apt install -y protobuf-compiler
+RUN dart pub global activate protoc_plugin
+
+# Generate protobuf files
+RUN mkdir -p lib/src/generated
+RUN protoc --plugin=protoc-gen-dart=$HOME/.pub-cache/bin/protoc-gen-dart --proto_path=/app/proto/helloworld --dart_out=grpc:lib/src/generated -Iproto /app/proto/helloworld/helloworld.proto
+
+# AOT compile the binary
+RUN dart compile exe bin/server.dart -o bin/server
+
+# ENTRYPOINT [ "/usr/bin/dart", "/app/bin/server.dart" ]
+
+# Build minimal serving image from AOT-compiled `/server` and required system
+# libraries and configuration files stored in `/runtime/` from the build stage.
+FROM scratch
+COPY --from=build /runtime/ /
+COPY --from=build /app/bin/server /app/bin/
+
+# Start server.
+EXPOSE 50051
 CMD ["/app/bin/server"]