Docker: improve build cache reuse (#40351)

* Docker: improve build cache reuse

* Tests: cover Docker build cache layout

* Docker: fix sandbox cache mount continuations

* Docker: document qr-import manifest scope

* Docker: narrow e2e install inputs

* CI: cache Docker builds in workflows

* CI: route sandbox smoke through setup script

* CI: keep sandbox smoke on script path

Dockerfile.sandbox-browser

@@ -1,8 +1,12 @@
+# syntax=docker/dockerfile:1.7
+
 FROM debian:bookworm-slim@sha256:98f4b71de414932439ac6ac690d7060df1f27161073c5036a7553723881bffbe
 
 ENV DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get update \
+RUN --mount=type=cache,id=openclaw-sandbox-bookworm-apt-cache,target=/var/cache/apt,sharing=locked \
+  --mount=type=cache,id=openclaw-sandbox-bookworm-apt-lists,target=/var/lib/apt,sharing=locked \
+  apt-get update \
   && apt-get install -y --no-install-recommends \
     bash \
     ca-certificates \
@@ -17,8 +21,7 @@ RUN apt-get update \
     socat \
     websockify \
     x11vnc \
-    xvfb \
-  && rm -rf /var/lib/apt/lists/*
+    xvfb
 
 COPY --chmod=755 scripts/sandbox-browser-entrypoint.sh /usr/local/bin/openclaw-sandbox-browser