add Docker deployment with Alpine image, release config and health check

- Alpine multi-stage Dockerfile (131 MB image)
- Release overlays (bin/server, bin/migrate), env.sh, Release module
- Health check endpoint at GET /health
- Fly.io config with SQLite volume mount
- Fix hardcoded paths in optimizer.ex and variant_cache.ex to use
  Application.app_dir/2 (breaks in releases where Plug.Static serves
  from a different directory than CWD)
- strip_beams: true in release config
- Optimised .dockerignore and .gitignore for mockup variants

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/controllers/health_controller.ex

@@ -0,0 +1,7 @@
+defmodule SimpleshopThemeWeb.HealthController do
+  use SimpleshopThemeWeb, :controller
+
+  def show(conn, _params) do
+    json(conn, %{status: "ok"})
+  end
+end

lib/simpleshop_theme_web/router.ex

@@ -52,6 +52,13 @@ defmodule SimpleshopThemeWeb.Router do
     post "/checkout", CheckoutController, :create
   end
 
+  # Health check (no auth, no theme loading — for load balancers and uptime monitors)
+  scope "/", SimpleshopThemeWeb do
+    pipe_through [:api]
+
+    get "/health", HealthController, :show
+  end
+
   # Cart API (session persistence for LiveView)
   scope "/api", SimpleshopThemeWeb do
     pipe_through [:browser]