added docker container

Dockerfile

@@ -0,0 +1,17 @@
+FROM dart:stable-sdk
+
+WORKDIR /app
+
+# sqlite3 package uses system libsqlite3 at runtime.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends libsqlite3-0 curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY pubspec.yaml pubspec.lock ./
+RUN dart pub get
+
+COPY . .
+
+EXPOSE 8080
+
+CMD ["dart", "run", "bin/vpn_server.dart"]