Update build args for containerized engine to target linux.

blimp/docs/build.md

 # Using GN
 Blimp only supports building using [GN](../../tools/gn/README.md). A quick
 overview over how to use GN can be found in the GN
 [quick start guide](../../tools/gn/docs/quick_start.md).
 
 There are three different build configurations depending on what you want to
 build:
 
 ## Android client
 
@@ skipping 24 matching lines @@
 You can also build and install incremental APK like this:
 
 ```bash
 ninja -C out-android/Debug blimp blimp_apk_incremental &&
     out-android/Debug/bin/install_blimp_apk_incremental
 ```
 
 ## Engine inside a Docker container
 
 Create another out-directory and set the GN args. Note, when building to run
-inside a [Docker container](container.md) you'll need to set the target_os to "chromeos":
+inside a [Docker container](container.md) you'll need to turn off several
+dependencies:
 
 ```bash
-mkdir -p out-chromeos/Debug
-gn args out-chromeos/Debug
+mkdir -p out-docker/Debug
+gn args out-docker/Debug
 ```
 
 This will bring an editor, where you can type in the following:
 
 ```bash
-target_os = "chromeos"
+target_os = "linux"
 is_debug = true
 is_clang = true
 symbol_level = 1  # Use -g1 instead of -g2
 use_goma = true
 use_aura = true
 use_ozone = true
 use_alsa = false
 use_pulseaudio = false
+use_cups = false
+use_glib = false
 ```
 
 To build:
 
 ```bash
-ninja -C out-chromeos/Debug blimp
+ninja -C out-docker/Debug blimp
 ```
 
 ## "Bare" engine, no Docker container
 
 Create another out-directory and set the GN args:
 
 ```bash
 mkdir -p out-linux/Debug
 gn args out-linux/Debug
 ```
 
 This will bring an editor, where you can type in the following:
 
 ```bash
 is_debug = true
 is_clang = true
 is_component_build = true
 symbol_level = 1  # Use -g1 instead of -g2
 use_goma = true
 use_aura = true
 use_ozone = true
 ```
 
 To build:
 
 ```bash
 ninja -C out-linux/Debug blimp
 ```