replace debian with alpine in the docker demo (starts much faster)

README.md

@@ -544,9 +544,9 @@ Try Powerlevel10k in Docker. You can safely make any changes to the file system
 the theme. Once you exit Zsh, the image is deleted.
 
 ```zsh
-docker run -e TERM -e COLORTERM -it --rm debian:buster bash -uec '
+docker run -e TERM -e COLORTERM -it --rm alpine sh -uec '
-  apt update
+  apk update
-  apt install -y git zsh nano vim
+  apk add git zsh nano vim
   git clone --depth=1 https://github.com/romkatv/powerlevel10k.git ~/powerlevel10k
   echo "source ~/powerlevel10k/powerlevel10k.zsh-theme" >>~/.zshrc
   cd ~/powerlevel10k