ci: Install sccache by using existing script

Author: tesuji

ci/docker/x86_64-unknown-linux-musl/Dockerfile

@@ -1,13 +1,15 @@
 FROM ubuntu:18.04
 
-RUN apt-get update && apt-get install -y \
-  musl-dev \
-  musl-tools \
-  curl \
-  ca-certificates \
-  perl \
-  make \
-  gcc
+COPY ci/docker/scripts/sccache.sh /scripts/
 
-RUN curl -L https://github.com/mozilla/sccache/releases/download/0.2.8/sccache-0.2.8-x86_64-unknown-linux-musl.tar.gz | tar xzf - \
-  && mv ./sccache-0.2.8-x86_64-unknown-linux-musl/sccache /usr/bin/sccache && rm -rf ./sccache-0.2.8-x86_64-unknown-linux-musl
+RUN \
+  apt-get update && \
+  apt-get install -qy \
+    musl-dev \
+    musl-tools \
+    curl \
+    ca-certificates \
+    perl \
+    make \
+    gcc && \
+  sh /scripts/sccache.sh