fix: restore multi-stage Dockerfiles, use cameleer-docker-builder

Follow cameleer3-server CI pattern: docker job uses
cameleer-docker-builder:1 (has Docker CLI), Dockerfiles contain
multi-stage builds (self-contained, no external toolchain needed).

- Dockerfile: restore frontend + maven + runtime stages
- ui/sign-in/Dockerfile: add node build stage + Logto base
- ci.yml: docker job reverts to cameleer-docker-builder:1,
  passes REGISTRY_TOKEN as build-arg, adds build cache

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

ui/sign-in/Dockerfile

@@ -1,2 +1,10 @@
+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
+COPY . .
+RUN npm run build
+
 FROM ghcr.io/logto-io/logto:latest
-COPY dist/ /etc/logto/packages/experience/dist/
+COPY --from=build /ui/dist/ /etc/logto/packages/experience/dist/