README: Update the developer image build instructions

The base Docker image is the one in which the user and group IDs can be
specified, so all the Docker images must be built in row to generate a
developer image with custom user and group IDs.

Signed-off-by: Adrien Ricciardi <aricciardi@baylibre.com>

Author: RICCIARDI-Adrien

README.md

@@ -63,10 +63,12 @@ docker run -ti -v $HOME/Work/zephyrproject:/workdir \
 
 #### Building Developer Docker Image
 
-The developer docker image can be built using the following command:
+The developer docker image can be built using the following commands:
 
 ```
-docker build -f Dockerfile.devel --build-arg UID=$(id -u) --build-arg GID=$(id -g) -t zephyr-build:v<tag> .
+docker build -f Dockerfile.base --build-arg UID=$(id -u) --build-arg GID=$(id -g) -t zephyr-ci-base:v<tag> .
+docker build -f Dockerfile.ci --build-arg BASE_IMAGE=zephyr-ci-base:v<tag> -t zephyr-ci:v<tag> .
+docker build -f Dockerfile.devel --build-arg BASE_IMAGE=zephyr-ci:v<tag> -t zephyr-build:v<tag> .
 ```
 
 It can be used for building Zephyr samples and tests by mounting the Zephyr workspace into it: