chore: update Dockerfile to set PNPM_HOME environment variable and prepare directory for pnpm installation

Dockerfile

@@ -3,12 +3,16 @@
 # ---- Base ----
 FROM node:22-alpine AS base
 RUN corepack enable && corepack prepare pnpm@latest --activate
+ENV PNPM_HOME="/pnpm"
+ENV PATH="$PNPM_HOME:$PATH"
+RUN mkdir -p $PNPM_HOME
 WORKDIR /app
 
 # ---- Dependencies ----
 FROM base AS deps
 COPY package.json pnpm-lock.yaml ./
-RUN pnpm install --frozen-lockfile
+RUN --mount=type=cache,id=pnpm,target=/pnpm/store \
+    pnpm install --frozen-lockfile
 
 # ---- Build ----
 FROM base AS builder