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>

fly.toml

@@ -0,0 +1,50 @@
+# fly.toml — Fly.io deployment configuration
+#
+# Getting started:
+#   1. Install flyctl: curl -L https://fly.io/install.sh | sh
+#   2. Sign up / log in: fly auth login
+#   3. Create the app: fly launch --no-deploy
+#   4. Create a volume: fly volumes create simpleshop_data --size 1
+#   5. Set secrets:
+#        fly secrets set SECRET_KEY_BASE=$(mix phx.gen.secret)
+#        fly secrets set PHX_HOST=your-domain.com
+#   6. Deploy: fly deploy
+#   7. Open: fly open
+
+app = "simpleshop-theme"
+primary_region = "lhr"
+
+[build]
+  dockerfile = "Dockerfile"
+
+# SQLite needs a persistent volume — without this, your database is lost on every deploy.
+[mounts]
+  source = "simpleshop_data"
+  destination = "/data"
+
+[env]
+  PHX_SERVER = "true"
+  DATABASE_PATH = "/data/simpleshop_theme.db"
+
+[http_service]
+  internal_port = 4000
+  force_https = true
+  auto_stop_machines = "stop"
+  auto_start_machines = true
+  min_machines_running = 0
+
+  [http_service.concurrency]
+    type = "connections"
+    hard_limit = 1000
+    soft_limit = 1000
+
+[[http_service.checks]]
+  grace_period = "30s"
+  interval = "15s"
+  method = "GET"
+  timeout = "5s"
+  path = "/health"
+
+[[vm]]
+  size = "shared-cpu-1x"
+  memory = "1gb"