build.md: Document --build-arg without value
This use case is currently _working correctly_, which is nice, but there is no documentation to be found about it. This PR fixes that.
@@ -425,6 +425,18 @@ Dockerfile are echoed during the build process.
For detailed information on using `ARG` and `ENV` instructions, see the
[Dockerfile reference](../builder.md).
+You may also use the `--build-arg` flag without a value, in which case the value
+from the local environment will be propagated into the Docker container being
+built:
+
+```bash
+$ export HTTP_PROXY=http://10.20.30.2:1234
+$ docker build --build-arg HTTP_PROXY .
+```
+
+This is similar to how `docker run -e` works. Refer to the [`docker run` documentation](https://docs.docker.com/engine/reference/commandline/run/#set-environment-variables--e---env---env-file)
+for more information.
+
### Optional security options (--security-opt)
This flag is only supported on a daemon running on Windows, and only supports
