docs: use environment variable for argo cd user (argoproj#12024)

Signed-off-by: rumstead <37445536+rumstead@users.noreply.github.com>

Signed-off-by: rumstead <37445536+rumstead@users.noreply.github.com>
Signed-off-by: schakrad <chakradari.sindhu@gmail.com>

Author: rumstead

docs/operator-manual/custom_tools.md

@@ -69,5 +69,5 @@ RUN apt-get update && \
     chmod +x /usr/local/bin/sops
 
 # Switch back to non-root user
-USER 999
+USER $ARGOCD_USER_ID
 ```