docs: add example kaniko cache warmer script (#123)

Author: johnstcn

README.md

@@ -188,6 +188,8 @@ docker run -it --rm \
 
 In Kubernetes, you can pre-populate a persistent volume with the same warmer image, then mount it into many workspaces with the [`ReadOnlyMany` access mode](https://kubernetes.io/docs/concepts/storage/persistent-volumes/#access-modes).
 
+A sample script to pre-fetch a number of images can be viewed [here](./examples/kaniko-cache-warmer.sh). This can be run, for example, as a cron job to periodically fetch the latest versions of a number of base images.
+
 ## Setup Script
 
 The `SETUP_SCRIPT` environment variable dynamically configures the user and init command (PID 1) after the container build process.

examples/kaniko-cache-warmer.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+# This is an example script to pull a number of images into the Kaniko cache
+# to have them ready for consumption by envbuilder.
+# Ref: https://github.com/coder/envbuilder/blob/main/README.md#image-caching
+KANIKO_CACHE_VOLUME=${KANIKO_CACHE_VOLUME:-"kanikocache"}
+IMAGES=(
+    alpine:latest
+    debian:latest
+    ubuntu:latest
+)
+
+set -euo pipefail
+
+if ! docker volume inspect "${KANIKO_CACHE_VOLUME}" > /dev/null 2>&1; then
+    echo "Kaniko cache volume does not exist; creating it."
+    docker volume create "${KANIKO_CACHE_VOLUME}"
+fi
+
+for img in "${IMAGES[@]}"; do
+    echo "Fetching image ${img} to kaniko cache ${KANIKO_CACHE_VOLUME}"
+    docker run --rm \
+        -v "${KANIKO_CACHE_VOLUME}:/cache" \
+        gcr.io/kaniko-project/warmer:latest \
+            --cache-dir=/cache \
+            --image="${img}"
+done