fix: use BUILDPLATFORM for native cross-compilation, remove broken cache mounts

Build and frontend stages now use --platform=$BUILDPLATFORM so Maven and
Node run natively on the ARM64 CI runner instead of under QEMU emulation.
Only the final JRE runtime stage targets amd64. Removed --mount=type=cache
which doesn't persist across CI runs with buildx --push; the registry layer
cache (--cache-from/--cache-to in CI) handles caching the dependency layer.

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

Dockerfile

@@ -1,5 +1,7 @@
 # syntax=docker/dockerfile:1
-FROM node:22-alpine AS frontend
+
+# Frontend: runs natively on build host
+FROM --platform=$BUILDPLATFORM node:22-alpine AS frontend
 ARG REGISTRY_TOKEN
 WORKDIR /ui
 COPY ui/package.json ui/package-lock.json ui/.npmrc ./
@@ -7,15 +9,18 @@ RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_T
 COPY ui/ .
 RUN npm run build
 
-FROM eclipse-temurin:21-jdk-alpine AS build
+# Maven build: runs natively on build host (no QEMU emulation)
+FROM --platform=$BUILDPLATFORM eclipse-temurin:21-jdk-alpine AS build
 WORKDIR /build
 COPY .mvn/ .mvn/
 COPY mvnw pom.xml ./
-RUN --mount=type=cache,target=/root/.m2/repository ./mvnw dependency:go-offline -B
+# Cache deps — only re-downloaded when POM changes
+RUN ./mvnw dependency:go-offline -B || true
 COPY src/ src/
 COPY --from=frontend /src/main/resources/static/ src/main/resources/static/
-RUN --mount=type=cache,target=/root/.m2/repository ./mvnw package -DskipTests -B
+RUN ./mvnw package -DskipTests -B
 
+# Runtime: target platform (amd64)
 FROM eclipse-temurin:21-jre-alpine
 WORKDIR /app
 RUN addgroup -S cameleer && adduser -S cameleer -G cameleer