README: Point to 'main' image tag instead of 'latest'

In the installation instructions, point to the 'main' image tag, which
is built from the 'main' branch, instead of the 'latest' tag which may
not necessarily point to the latest build in the 'main' branch.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

README.md

@@ -31,14 +31,14 @@ DockerHub (`docker.io`).
 
 ```
 docker run -ti -v $HOME/Work/zephyrproject:/workdir \
-           ghcr.io/zephyrproject-rtos/zephyr-build:latest
+           ghcr.io/zephyrproject-rtos/zephyr-build:main
 ```
 
 **DockerHub (`docker.io`)**
 
 ```
 docker run -ti -v $HOME/Work/zephyrproject:/workdir \
-           docker.io/zephyrprojectrtos/zephyr-build:latest
+           docker.io/zephyrprojectrtos/zephyr-build:main
 ```
 
 #### Building Developer Docker Image