Update docker/README.md to follow build step within container instance

Author: 0405ysj

docker/README.md

@@ -25,11 +25,16 @@ Please refer to
 
 ## Build docker image manually
 
-Please run below command to build manually.
+To build docker image, building host debian packages for docker image is
+required. Please refer to
+[tools/buildutils/cw/README.md](../tools/buildutils/cw/README.md) for building
+host debian packages including `base` and `frontend`.
+
+After retrieving host debian packages, please run below command to build
+manually.
 
 ```bash
 cd /path/to/android-cuttlefish
-tools/buildutils/build_packages.sh
 docker/image-builder.sh -m dev
 ```
 

tools/buildutils/cw/README.md

@@ -6,14 +6,16 @@
 
 The build image command must be run at the root of the `android-cuttlefish` repo directory.
 
+Enabling Docker [BuildKit](https://docs.docker.com/build/buildkit/) is required
+on Docker version below 23.0 to build this image.
+
 ```
 docker build \
   --file "tools/buildutils/cw/Containerfile" \
   --tag "android-cuttlefish-build:latest" \
   .
 ```
 
-
 ## Build the package
 
 The run container command must be run at the root of the `android-cuttlefish` repo directory.