feat(bonsai): invalidation sélective du cache via CACHEBUST

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

bonsai/Dockerfile.webapp

@@ -2,6 +2,7 @@
 
 FROM node:22-alpine AS builder
 RUN apk add --no-cache git openssh
+ARG CACHEBUST
 RUN --mount=type=secret,id=ssh_key \
     --mount=type=cache,target=/root/.npm \
     mkdir -p /root/.ssh && \