switch to debian in the docker demo

archlinux fails to install packages half the time

  error: key "Christian Hesse (Arch Linux Package Signing) <arch@eworm.de>" could not be imported

This seems to be a long-standing bug in arch that hasn't been acknowledged.
Users suffering from it are offered to to repeatedly wipe their keyring,
reinitialize, and retry.

README.md

@@ -176,8 +176,9 @@ 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 -it --rm archlinux/base bash -uexc '
-  pacman -Sy --noconfirm zsh git
+docker run -e TERM -it --rm debian:buster bash -uec '
+  apt update
+  apt install -y git zsh
   git clone --depth=1 https://github.com/romkatv/powerlevel10k.git ~/powerlevel10k
   echo "source ~/powerlevel10k/powerlevel10k.zsh-theme" >>~/.zshrc
   cd ~/powerlevel10k