use shared scripts for init and sccache in cross image

src/ci/docker/cross/Dockerfile

@@ -21,14 +21,9 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libssl-dev \
   pkg-config
 
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-05-12-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
-
-RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
-    dpkg -i dumb-init_*.deb && \
-    rm dumb-init_*.deb
-ENTRYPOINT ["/usr/bin/dumb-init", "--"]
+# dumb-init
+COPY scripts/dumb-init.sh /scripts/
+RUN sh /scripts/dumb-init.sh
 
 WORKDIR /tmp
 
@@ -80,3 +75,10 @@ ENV RUST_CONFIGURE_ARGS \
       --musl-root-armhf=/usr/local/arm-linux-musleabihf \
       --musl-root-armv7=/usr/local/armv7-linux-musleabihf
 ENV SCRIPT python2.7 ../x.py dist --target $TARGETS
+
+# sccache
+COPY scripts/sccache.sh /scripts/
+RUN sh /scripts/sccache.sh
+
+# init
+ENTRYPOINT ["/usr/bin/dumb-init", "--"]