fix: install pnpm via npm instead of corepack

Alpine npm package doesn't include corepack. Install pnpm globally via npm instead.

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

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

Dockerfile

@@ -2,10 +2,7 @@
 FROM ruby:3.4.1-alpine AS builder
 
 # Install build dependencies
-RUN apk add --no-cache build-base nodejs npm
-
-# Enable corepack for pnpm
-RUN corepack enable
+RUN apk add --no-cache build-base nodejs npm && npm install -g pnpm
 
 # Set working directory
 WORKDIR /app