feat(bonsai): invalidation sélective du cache via CACHEBUST

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

bonsai/Dockerfile.api

@@ -2,6 +2,7 @@
 
 FROM eclipse-temurin:25-jdk AS builder
 RUN apt-get update && apt-get install -y git openssh-client && rm -rf /var/lib/apt/lists/*
+ARG CACHEBUST
 RUN --mount=type=secret,id=ssh_key \
     --mount=type=cache,target=/root/.gradle \
     mkdir -p /root/.ssh && \