feat(docker): containerize site with configurable host port

Add Dockerfile (nginx:1.27-alpine), nginx.conf (gzip, cache, CSP and
security headers, no HSTS — left to outer proxy), and docker-compose
service `bchanot-web`. Host port is configurable via PORT env var
(default 8080) and bound to 127.0.0.1 so the container sits behind a
reverse proxy. Container hardened with read_only fs, cap_drop ALL,
no-new-privileges, and tmpfs for nginx runtime dirs. Healthcheck via
wget on /. Also adds .dockerignore and .env.example, and ignores .env.

Usage:
  cp .env.example .env
  docker compose up -d --build

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

.dockerignore

@@ -0,0 +1,20 @@
+# Source control / Claude / local-only
+.git
+.gitignore
+.claude
+CLAUDE.md
+README.md
+
+# Docker meta (not needed inside the image)
+Dockerfile
+docker-compose.yml
+.dockerignore
+.env
+.env.example
+nginx.conf.bak
+
+# Editor / OS noise
+*.swp
+.DS_Store
+node_modules
+graphify-out