add docs/example for Artifact Registry pull-through cache (#208)

Signed-off-by: Jason Hall <jason@chainguard.dev>

gcp-artifact-registry/README.md

@@ -0,0 +1,55 @@
+# Configuring Google Artifact Registry Pull-Through Cache for cgr.dev
+
+This directory contains Terraform resources to configure a [Google Artifact Registry remote repository](https://cloud.google.com/artifact-registry/docs/repositories/remote-repo) for a private `cgr.dev` registry, using a [Pull Token](https://edu.chainguard.dev/chainguard/chainguard-registry/authenticating/#authenticating-with-a-pull-token).
+
+First, create a pull token for your organization:
+
+```
+chainctl auth configure-docker --pull-token
+```
+
+This will print a `docker login` command that you can use to authenticate with the pull token. For example:
+
+```
+To use this pull token in another environment, run this command:
+
+    docker login "cgr.dev" --username "abcdef11e47a927f78ffa39cd6393f4a83ec6eb7/e3b0123abcdef123" --password "eyJh...krglE"
+```
+
+We'll use this username and password to configure the pull-through cache in Terraform.
+
+Next, `terraform init` and `terraform apply` the configuration in this directory:
+
+```
+terraform apply
+```
+
+This will prompt for your GCP project, and the Chainguard pull token `username` and `password`.
+
+When Terraform is done creating resources, it will print the remote repo URLs that were created.
+
+```
+repos = [
+  "us-east4-docker.pkg.dev/my-cool-project/remote",
+]
+```
+
+You can now pull through these URLs to pull images from the `cgr.dev` registry.
+
+Simply append your Chainguard organization name and repo name to the URL, and use it as a remote repo URL.
+
+```
+docker pull us-east4-docker.pkg.dev/my-cool-project/remote/customer.biz/image:latest
+latest: Pulling from my-cool-project/remote/customer.biz/image
+6cdb74e61124: Downloading [====>                                              ]  18.21MB/224.2MB
+```
+
+You can configure Terraform to set up resources in other regions, or with other names, by updating these variables:
+
+```
+TF_VAR_regions="['us-central1']" TF_VAR_repo="custom" terraform apply
+...
+repos = [
+  "us-central1-docker.pkg.dev/my-cool-project/custom",
+]
+```

gcp-artifact-registry/main.tf

@@ -0,0 +1,88 @@
+variable "project" {
+  type        = string
+  description = "The GCP project ID"
+}
+
+variable "repo" {
+  type        = string
+  description = "The GCP Artifact Registry repository name"
+  default     = "remote"
+}
+
+variable "regions" {
+  type        = list(string)
+  description = "The GCP regions to deploy to"
+  default     = ["us-east4"]
+}
+
+variable "username" {
+  type        = string
+  description = "Username for the cgr.dev pull token"
+}
+
+variable "password" {
+  sensitive   = true
+  type        = string
+  description = "Password for the cgr.dev pull token"
+}
+
+provider "google" {
+  project = var.project
+}
+
+resource "google_secret_manager_secret" "pull-token" {
+  secret_id = "pull-token"
+  replication {
+    auto {}
+  }
+}
+
+resource "google_secret_manager_secret_version" "pull-token" {
+  secret      = google_secret_manager_secret.pull-token.name
+  secret_data = var.password
+}
+
+resource "google_artifact_registry_repository" "repo" {
+  for_each = toset(var.regions)
+
+  location      = each.key
+  repository_id = var.repo
+  description   = "Remote Chainguard Images repository"
+  format        = "DOCKER"
+  mode          = "REMOTE_REPOSITORY"
+  remote_repository_config {
+    description = "chainguard"
+
+    // The password won't be populated at first, so we need to disable
+    // validating the token at repo-creation time.
+    disable_upstream_validation = true
+
+    docker_repository {
+      custom_repository {
+        uri = "https://cgr.dev"
+      }
+    }
+
+    upstream_credentials {
+      username_password_credentials {
+        username                = var.username
+        password_secret_version = google_secret_manager_secret_version.pull-token.name
+      }
+    }
+  }
+}
+
+data "google_project" "project" {}
+
+locals { project-number = data.google_project.project.number }
+
+// Give the AR service account the ability to read the secret.
+resource "google_secret_manager_secret_iam_member" "reader" {
+  secret_id = google_secret_manager_secret.pull-token.id
+  role      = "roles/secretmanager.secretAccessor"
+  member    = "serviceAccount:service-${local.project-number}@gcp-sa-artifactregistry.iam.gserviceaccount.com"
+}
+
+output "repos" {
+  value = [for r in var.regions : "${google_artifact_registry_repository.repo[r].location}-docker.pkg.dev/${var.project}/${google_artifact_registry_repository.repo[r].name}"]
+}