CI: Enable BuildKit cache mounts for faster npm installs

- Add syntax directive for BuildKit cache-mount support
- Cache /root/.npm between builds (web + server npm install)
- Enable DOCKER_BUILDKIT=1 in CI build step
- Remove aggressive builder prune that killed layer cache after each build

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,7 +1,9 @@
+# syntax=docker/dockerfile:1
+
 FROM node:24-slim AS web-build
 WORKDIR /app/web
 COPY web/package*.json ./
-RUN npm install --no-audit --no-fund
+RUN --mount=type=cache,target=/root/.npm npm install --no-audit --no-fund
 COPY web/ .
 ARG VITE_BUILD_CHANNEL=stable
 ARG VITE_APP_VERSION=dev
@@ -13,7 +15,7 @@ FROM node:24-slim AS server-deps
 WORKDIR /app/server
 RUN apt-get update && apt-get install -y python3 make g++ && rm -rf /var/lib/apt/lists/*
 COPY server/package*.json ./
-RUN npm install --no-audit --no-fund
+RUN --mount=type=cache,target=/root/.npm npm install --no-audit --no-fund
 
 FROM server-deps AS server-build
 COPY server/ .