docs: Update example dockerfile (argoproj#11721)

The latest tag hasn't been updated in almost a year, and as a result, the ubuntu repositories are out of date and are throwing errors. This updates the example to use a fixed version, which are updated much more frequently.

Signed-off-by: Phil Wright- Christie <philwc@gmail.com>

Signed-off-by: Phil Wright- Christie <philwc@gmail.com>
Co-authored-by: Michael Crenshaw <350466+crenshaw-dev@users.noreply.github.com>
Signed-off-by: schakrad <chakradari.sindhu@gmail.com>

docs/operator-manual/custom_tools.md

@@ -51,7 +51,7 @@ following example builds an entirely customized repo-server from a Dockerfile, i
 dependencies that may be needed for generating manifests.
 
 ```Dockerfile
-FROM argoproj/argocd:latest
+FROM argoproj/argocd:v2.5.4 # Replace tag with the appropriate argo version
 
 # Switch to root for the ability to perform install
 USER root