feat: add Dockerfile for Coolify deployment

Multi-stage build:
- Stage 1: Build Jekyll site with Ruby 3.4.1
- Stage 2: Serve with nginx:alpine

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Dockerfile

@@ -0,0 +1,30 @@
+# Build stage
+FROM ruby:3.4.1-alpine AS builder
+
+# Install build dependencies
+RUN apk add --no-cache build-base
+
+# Set working directory
+WORKDIR /app
+
+# Copy Gemfile and install dependencies
+COPY Gemfile Gemfile.lock ./
+RUN bundle install
+
+# Copy source files
+COPY . .
+
+# Build Jekyll site
+RUN bundle exec jekyll build
+
+# Production stage
+FROM nginx:alpine
+
+# Copy built site from builder
+COPY --from=builder /app/_site /usr/share/nginx/html
+
+# Expose port 80
+EXPOSE 80
+
+# Start nginx
+CMD ["nginx", "-g", "daemon off;"]