chore: replace nixpacks with Dockerfile for faster cached builds

.dockerignore

@@ -0,0 +1,6 @@
+node_modules
+dist
+.git
+.gitignore
+*.md
+.env*

Dockerfile

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+# ---- Stage 1: deps (only invalidated when lockfile changes) ----
+FROM node:22-alpine AS deps
+
+RUN corepack enable && corepack prepare pnpm@latest --activate
+
+WORKDIR /app
+
+COPY package.json pnpm-lock.yaml ./
+
+RUN pnpm install --frozen-lockfile
+
+
+# ---- Stage 2: build (invalidated on any source change) ----
+FROM node:22-alpine AS builder
+
+RUN corepack enable && corepack prepare pnpm@latest --activate
+
+WORKDIR /app
+
+COPY --from=deps /app/node_modules ./node_modules
+
+COPY . .
+
+RUN pnpm run build
+
+
+# ---- Stage 3: serve ----
+FROM nginx:alpine AS runner
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=3s \
+  CMD wget -qO- http://localhost:80/ || exit 1