feat(docker): production multi-stage Dockerfile

Three named targets (app, horizon, scheduler) sharing a runtime-base
with PHP 8.3-FPM, opcache, redis, and pinned php-fpm pool config.
Composer + Node build stages are separate so vendor/ and public/build/
are baked into the runtime image.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,30 @@
+# .dockerignore
+.git
+.gitignore
+.github
+.gitea
+.idea
+.vscode
+.claude
+.superpowers
+.playwright-mcp
+docker
+!docker/prod-entrypoint.sh
+docker-compose*.yml
+.env*
+!website/.env.example
+docs
+helm
+node_modules
+website/node_modules
+website/vendor
+website/storage/logs/*
+website/storage/framework/cache/data/*
+website/storage/framework/sessions/*
+website/storage/framework/views/*
+website/.phpunit.cache
+website/tests
+*.md
+Makefile
+scripts/whmcs-migrate
+ipv4-outreach-tickets.txt