Create gn arg templates for use with blimp builds. Update build doc.

build/args/blimp_engine_containerized.gn

Index: build/args/blimp_engine_containerized.gn
diff --git a/build/args/blimp_engine_containerized.gn b/build/args/blimp_engine_containerized.gn
new file mode 100644
index 0000000000000000000000000000000000000000..152652c725dc77339716600b9462a1818fc6796d
--- /dev/null
+++ b/build/args/blimp_engine_containerized.gn
@@ -0,0 +1,16 @@
+# GN args template for a blimp engine to be run within a docker container.
+#
+# Copy to arg.gn in out directory and run gn gen on the directory to use.
+# E.g. for out directory out/foo:
+# echo "import(\"build/args/blimp_engine_containerized.gn\")" > out/foo/args.gn
+# gn gen out/foo
+#
+# Use gn args to add your own build preference args.
+
+import("//build/args/blimp_engine.gn")
+
+# Not available within docker container.
+use_alsa = false
+use_pulseaudio = false
+use_cups = false
+use_glib = false