Update Dockerfile (#68)

Author: jerson

example/Dockerfile

@@ -3,9 +3,9 @@ FROM jerson/flutter:web AS build-env
 WORKDIR  /app
 COPY . .
 RUN flutter upgrade
-RUN cd example && flutter build web
+RUN cd example && flutter build web --release
 
 FROM nginx
 
 COPY --from=build-env /app/example/docker/nginx.conf /etc/nginx/nginx.conf
-COPY --from=build-env /app/example/build/web /usr/share/nginx/html
+COPY --from=build-env /app/example/build/web /usr/share/nginx/html