feat: add base templates for all major cloud providers (#26634)

Add 6 new base templates to the template builder, covering all major
cloud providers and platforms:

- **scratch**: Minimal starter template with only `coder_agent` and
metadata
- **aws-windows**: AWS EC2 Windows instances with PowerShell user_data
- **azure-linux**: Azure VMs with cloud-init and managed disk
persistence
- **gcp-linux**: Google Compute Engine Linux instances with persistent
disk
- **gcp-windows**: Google Compute Engine Windows instances
- **digitalocean-linux**: DigitalOcean Linux droplets with persistent
volumes

Each base includes `base.json`, `main.tf.tmpl`, `README.md` (with
prerequisite markers), and any static files (cloud-init configs). Tests
verify all 9 bases load, render without error, and produce valid
single-agent declarations.

`azure-windows` is deferred; it needs to be registered in the `examples`
package first.

Depends on #26633

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

---

NB: This is very much an agent-generated PR and draws completely from
base templates that exist in `examples/templates/`. The base.json files
are new, so review those, but don't spend any brain tokens on the
correctness of the terraform and supporting files: any issues there are
issues with the upstream example template

coderd/templatebuilder/bases/gcp-linux/README.md

@@ -0,0 +1,62 @@
+---
+display_name: Google Compute Engine (Linux)
+description: Provision Google Compute Engine instances as Coder workspaces
+icon: ../../../site/static/icon/gcp.png
+maintainer_github: coder
+verified: true
+tags: [vm, linux, gcp]
+---
+
+# Remote Development on Google Compute Engine (Linux)
+
+<!-- prerequisites:start -->
+
+## Prerequisites
+
+### Authentication
+
+This template assumes that coderd is run in an environment that is authenticated
+with Google Cloud. For example, run `gcloud auth application-default login` to
+import credentials on the system and user running coderd. For other ways to
+authenticate [consult the Terraform
+docs](https://registry.terraform.io/providers/hashicorp/google/latest/docs/guides/getting_started#adding-credentials).
+
+Coder requires a Google Cloud Service Account to provision workspaces. To create
+a service account:
+
+1. Navigate to the [CGP
+   console](https://console.cloud.google.com/projectselector/iam-admin/serviceaccounts/create),
+   and select your Cloud project (if you have more than one project associated
+   with your account)
+
+1. Provide a service account name (this name is used to generate the service
+   account ID)
+
+1. Click **Create and continue**, and choose the following IAM roles to grant to
+   the service account:
+
+   - Compute Admin
+   - Service Account User
+
+   Click **Continue**.
+
+1. Click on the created key, and navigate to the **Keys** tab.
+
+1. Click **Add key** > **Create new key**.
+
+1. Generate a **JSON private key**, which will be what you provide to Coder
+   during the setup process.
+
+<!-- prerequisites:end -->
+
+## Architecture
+
+This template provisions the following resources:
+
+- GCP VM (ephemeral)
+- GCP Disk (persistent, mounted to root)
+
+Coder persists the root volume. The full filesystem is preserved when the workspace restarts. See this [community example](https://github.com/bpmct/coder-templates/tree/main/aws-linux-ephemeral) of an ephemeral AWS instance.
+
+> **Note**
+> This template is designed to be a starting point! Edit the Terraform to extend the template to support your use case.

coderd/templatebuilder/bases/gcp-linux/base.json

@@ -0,0 +1,6 @@
+{
+	"id": "gcp-linux",
+	"display_name": "Google Compute Engine (Linux)",
+	"os": "linux",
+	"default_context": {}
+}

coderd/templatebuilder/bases/gcp-linux/main.tf.tmpl

@@ -0,0 +1,152 @@
+terraform {
+  required_providers {
+    coder = {
+      source = "coder/coder"
+    }
+    google = {
+      source = "hashicorp/google"
+    }
+  }
+}
+
+provider "coder" {}
+
+variable "project_id" {
+  description = "Which Google Compute Project should your workspace live in?"
+}
+
+# See https://registry.coder.com/modules/coder/gcp-region
+module "gcp_region" {
+  source = "registry.coder.com/coder/gcp-region/coder"
+
+  # This ensures that the latest non-breaking version of the module gets downloaded, you can also pin the module version to prevent breaking changes in production.
+  version = "~> 1.0"
+
+  regions = ["us", "europe"]
+  default = "us-central1-a"
+}
+
+provider "google" {
+  zone    = module.gcp_region.value
+  project = var.project_id
+}
+
+data "google_compute_default_service_account" "default" {}
+
+data "coder_workspace" "me" {}
+data "coder_workspace_owner" "me" {}
+
+resource "google_compute_disk" "root" {
+  name  = "coder-${data.coder_workspace.me.id}-root"
+  type  = "pd-ssd"
+  zone  = module.gcp_region.value
+  image = "debian-cloud/debian-11"
+  lifecycle {
+    ignore_changes = [name, image]
+  }
+}
+
+resource "coder_agent" "main" {
+  auth           = "google-instance-identity"
+  arch           = "amd64"
+  os             = "linux"
+  startup_script = <<-EOT
+    set -e
+
+    # Add any commands that should be executed at workspace startup (e.g install requirements, start a program, etc) here
+  EOT
+
+  metadata {
+    key          = "cpu"
+    display_name = "CPU Usage"
+    interval     = 5
+    timeout      = 5
+    script       = <<-EOT
+      #!/bin/bash
+      set -e
+      top -bn1 | grep "Cpu(s)" | awk '{print $2 + $4 "%"}'
+    EOT
+  }
+  metadata {
+    key          = "memory"
+    display_name = "Memory Usage"
+    interval     = 5
+    timeout      = 5
+    script       = <<-EOT
+      #!/bin/bash
+      set -e
+      free -m | awk 'NR==2{printf "%.2f%%\t", $3*100/$2 }'
+    EOT
+  }
+  metadata {
+    key          = "disk"
+    display_name = "Disk Usage"
+    interval     = 600 # every 10 minutes
+    timeout      = 30  # df can take a while on large filesystems
+    script       = <<-EOT
+      #!/bin/bash
+      set -e
+      df /home/coder | awk '$NF=="/"{printf "%s", $5}'
+    EOT
+  }
+}
+
+resource "google_compute_instance" "dev" {
+  zone         = module.gcp_region.value
+  count        = data.coder_workspace.me.start_count
+  name         = "coder-${lower(data.coder_workspace_owner.me.name)}-${lower(data.coder_workspace.me.name)}-root"
+  machine_type = "e2-medium"
+  network_interface {
+    network = "default"
+    access_config {
+      // Ephemeral public IP
+    }
+  }
+  boot_disk {
+    auto_delete = false
+    source      = google_compute_disk.root.name
+  }
+  service_account {
+    email  = data.google_compute_default_service_account.default.email
+    scopes = ["cloud-platform"]
+  }
+  # The startup script runs as root with no $HOME environment set up, so instead of directly
+  # running the agent init script, create a user (with a homedir, default shell and sudo
+  # permissions) and execute the init script as that user.
+  metadata_startup_script = <<EOMETA
+#!/usr/bin/env sh
+set -eux
+
+# If user does not exist, create it and set up passwordless sudo
+if ! id -u "${local.linux_user}" >/dev/null 2>&1; then
+  useradd -m -s /bin/bash "${local.linux_user}"
+  echo "${local.linux_user} ALL=(ALL) NOPASSWD:ALL" > /etc/sudoers.d/coder-user
+fi
+
+exec sudo -u "${local.linux_user}" sh -c '${coder_agent.main.init_script}'
+EOMETA
+}
+
+locals {
+  # Ensure Coder username is a valid Linux username
+  linux_user = lower(substr(data.coder_workspace_owner.me.name, 0, 32))
+}
+
+resource "coder_metadata" "workspace_info" {
+  count       = data.coder_workspace.me.start_count
+  resource_id = google_compute_instance.dev[0].id
+
+  item {
+    key   = "type"
+    value = google_compute_instance.dev[0].machine_type
+  }
+}
+
+resource "coder_metadata" "home_info" {
+  resource_id = google_compute_disk.root.id
+
+  item {
+    key   = "size"
+    value = "${google_compute_disk.root.size} GiB"
+  }
+}