perf: add BuildKit cache mounts for Maven and npm in Docker builds

Maven .m2 and npm caches persist across --no-cache builds, avoiding
full dependency re-downloads on every CI run.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/sign-in/Dockerfile

@@ -1,8 +1,10 @@
+# syntax=docker/dockerfile:1
+
 FROM --platform=$BUILDPLATFORM node:22-alpine AS build
 ARG REGISTRY_TOKEN
 WORKDIR /ui
 COPY package.json package-lock.json .npmrc ./
-RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_TOKEN}" >> .npmrc && npm ci
+RUN --mount=type=cache,target=/root/.npm echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_TOKEN}" >> .npmrc && npm ci
 COPY . .
 RUN npm run build