Add Docker development environment

Multistage Dockerfile (WooCommerce download, wp-bootstrap npm build,
Composer deps, WordPress runtime), Compose stack with MariaDB, and
auto-setup entrypoint that installs WordPress and activates the theme
on first boot.

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

.gitignore

@@ -31,3 +31,6 @@ npm-debug.log
 
 # Build artifacts (releases directory)
 releases/
+
+# Docker runtime
+.env