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>

config/runtime.exs

@@ -25,7 +25,7 @@ if config_env() == :prod do
     System.get_env("DATABASE_PATH") ||
       raise """
       environment variable DATABASE_PATH is missing.
-      For example: /etc/simpleshop_theme/simpleshop_theme.db
+      For example: /data/simpleshop_theme.db
       """
 
   config :simpleshop_theme, SimpleshopTheme.Repo,
@@ -97,22 +97,32 @@ if config_env() == :prod do
 
   # ## Configuring the mailer
   #
-  # In production you need to configure the mailer to use a different adapter.
-  # Here is an example configuration for Mailgun:
+  # Uncomment ONE of the blocks below and set the matching env vars.
+  # The API client is already configured in config/prod.exs (Swoosh.ApiClient.Req).
+  #
+  # Postmark (recommended):
+  #
+  #     config :simpleshop_theme, SimpleshopTheme.Mailer,
+  #       adapter: Swoosh.Adapters.Postmark,
+  #       api_key: System.get_env("POSTMARK_API_KEY")
+  #
+  # Mailgun:
   #
   #     config :simpleshop_theme, SimpleshopTheme.Mailer,
   #       adapter: Swoosh.Adapters.Mailgun,
   #       api_key: System.get_env("MAILGUN_API_KEY"),
   #       domain: System.get_env("MAILGUN_DOMAIN")
   #
-  # Most non-SMTP adapters require an API client. Swoosh supports Req, Hackney,
-  # and Finch out-of-the-box. This configuration is typically done at
-  # compile-time in your config/prod.exs:
+  # SMTP (any provider):
   #
-  #     config :swoosh, :api_client, Swoosh.ApiClient.Req
-  #
-  # See https://hexdocs.pm/swoosh/Swoosh.html#module-installation for details.
+  #     config :simpleshop_theme, SimpleshopTheme.Mailer,
+  #       adapter: Swoosh.Adapters.SMTP,
+  #       relay: System.get_env("SMTP_HOST"),
+  #       port: String.to_integer(System.get_env("SMTP_PORT") || "587"),
+  #       username: System.get_env("SMTP_USERNAME"),
+  #       password: System.get_env("SMTP_PASSWORD"),
+  #       tls: :if_available
 
-  # Stripe keys are stored encrypted in the database and loaded at runtime
-  # by SimpleshopTheme.Secrets. No env vars needed.
+  # Stripe and Printify keys are stored encrypted in the database and loaded
+  # at runtime by SimpleshopTheme.Secrets. No env vars needed for those.
 end