Update build.md
Explicitly stated that you must add --build-arg for each build argument.
Added multiple arguments to example of `--build-arg` usage.
Fix for
@@ -411,13 +411,13 @@ files. The `ARG` instruction lets Dockerfile authors define values that users
can set at build-time using the `--build-arg` flag:
```bash
-$ docker build --build-arg HTTP_PROXY=http://10.20.30.2:1234 .
+$ docker build --build-arg HTTP_PROXY=http://10.20.30.2:1234 --build-arg FTP_PROXY=http://40.50.60.5:4567 .
```
This flag allows you to pass the build-time variables that are
accessed like regular environment variables in the `RUN` instruction of the
Dockerfile. Also, these values don't persist in the intermediate or final images
-like `ENV` values do.
+like `ENV` values do. You must add `--build-arg` for each build argument.
Using this flag will not alter the output you see when the `ARG` lines from the
Dockerfile are echoed during the build process.
