docs: say build.network is a mode, not a Compose network (docker#25881)

build.network was showing `custom_network_1`, which looks like a
user-defined Compose network. it's the RUN networking mode (`default` /
`none` / `host`), same as `docker build --network`.

@netlify /reference/compose-file/build/

Fixes docker#21982

Signed-off-by: Dean Chen <862469039@qq.com>

Author: locker95

content/reference/compose-file/build.md

@@ -348,21 +348,19 @@ build:
 
 ### `network`
 
-Set the network containers connect to for the `RUN` instructions during build.
+Set the networking mode for `RUN` instructions during the build. This is
+the same as [`docker build --network`](/reference/cli/docker/buildx/build/#network).
+It is not a Compose network from the top-level [`networks`](networks.md) key.
 
-```yaml
-build:
-  context: .
-  network: host
-```
+The value is a mode (`default`, `none`, or `host`), not a network name:
 
 ```yaml
 build:
   context: .
-  network: custom_network_1
+  network: host
 ```
 
-Use `none` to disable networking during build:
+Use `none` to disable networking during the build:
 
 ```yaml
 build: