fix: add docker cache for pnpm installations

infrastructure/docker/Dockerfile.web

@@ -9,6 +9,10 @@ ARG NEXT_PUBLIC_API_URL=http://localhost:4000
 
 FROM node:24.15.0-alpine AS base
 RUN apk add --no-cache libc6-compat
+# Store pnpm's content-addressable store under PNPM_HOME so the BuildKit
+# `--mount=type=cache,target=/pnpm/store` cache actually persists deps across builds.
+ENV PNPM_HOME="/pnpm"
+ENV PATH="$PNPM_HOME:$PATH"
 RUN corepack enable
 WORKDIR /app