fix(ci): bound Docker build cache with BuildKit cache mounts

- Move the pnpm store, apk and .next caches into --mount=type=cache so
  dependencies are shared across builds instead of duplicated in fresh
  image layers (was the source of unbounded disk growth).
- Replace the deprecated --keep-storage prune flag in ci-deploy.sh with
  the working --max-used-space=4g (buildx v0.37 renamed the flag). The
  deprecated flag silently did nothing, so the BuildKit cache kept
  growing unbounded (was 15.86GB); it is now capped at 4GB after every
  deploy.

Dockerfile

@@ -4,22 +4,37 @@ FROM node:26.8.1-alpine AS migrations
 WORKDIR /app
 ENV NEXT_TELEMETRY_DISABLED=1
 # Keep the bootstrap aligned with package.json packageManager.
-RUN apk add --no-cache git \
+# The apk cache is persisted in a BuildKit cache mount so git is not
+# re-downloaded on every build.
+RUN --mount=type=cache,target=/var/cache/apk \
+    apk add --no-cache git \
     && npm install -g [email protected]
+# The pnpm store is kept in a BuildKit cache mount that persists across builds
+# on the builder. This is what stops disk usage from growing unbounded: the
+# downloaded dependency store is shared and reused instead of being copied into
+# a fresh image layer on every build. Unlike an image layer it is also prunable
+# independently, so a hard cap (see ci-deploy.sh) keeps it bounded.
+ENV PNPM_HOME=/pnpm PNPM_STORE=/pnpm/store
 # pnpm-workspace.yaml + .npmrc must be present too: the lockfile records the
 # overrides from pnpm-workspace.yaml, and --frozen-lockfile rejects a build
 # where the workspace config is absent (ERR_PNPM_LOCKFILE_CONFIG_MISMATCH).
 COPY package.json pnpm-lock.yaml* pnpm-workspace.yaml* .npmrc* ./
-# pnpm fetch: download all deps into $PNPM_STORE first, so only the lockfile
-# change (not source changes) invalidates the network-heavy download layer.
-RUN pnpm fetch --ignore-scripts
-RUN pnpm install --frozen-lockfile --ignore-scripts --offline
+# pnpm fetch: download all deps into the shared cache-mounted store.
+RUN --mount=type=cache,target=/pnpm \
+    pnpm fetch --ignore-scripts
+# Install offline from the cache-mounted store; the store itself stays in the
+# build cache between builds.
+RUN --mount=type=cache,target=/pnpm \
+    pnpm install --frozen-lockfile --ignore-scripts --offline
 COPY . .
 FROM migrations AS builder
 ARG NEXT_DEPLOYMENT_ID="unknown"
 ENV NEXT_DEPLOYMENT_ID="$NEXT_DEPLOYMENT_ID"
 # Fixture values exist only for this build command; production secrets are runtime-only.
-RUN DATABASE_URL="mysql://build:[email protected]:9/build" \
+# Cache Next.js build output and webpack caches so rebuilds only redo the
+# changed parts.
+RUN --mount=type=cache,target=/app/.next/cache \
+    DATABASE_URL="mysql://build:[email protected]:9/build" \
     HOTEL_NAME="Build fixture" APP_URL="http://localhost:3002" \
     AUTH_SECRET="build-fixture-not-for-runtime-use-000000000000" \
     pnpm run build