You can override the default mode for the command line by passing the --mode option argument, which defaults to "development".
By default – mode XX is not required to be written. As shown in the figure below; at
this time, it defaults to reading in .env.developmen. no. env.dev
If you want to read from .env.dev, you should customize it, just write dev after mode.
```"dev": "vue-cli-service build --mode dev"