Make cachebust parameter available as env var

This allows using an already built image that was built with cachebust
set without rebuilding osu.

Dockerfile

@@ -8,6 +8,7 @@ RUN apt-get update \
 COPY disable-deployed-build.patch /tmp/
 
 ARG CACHEBUST=1
+ENV CACHEBUST=$CACHEBUST
 
 RUN OSU_VERSION=$(curl -s https://api.github.com/repos/ppy/osu/releases/latest | jq -r .tag_name) \
     && git clone --depth=1 --single-branch -b ${OSU_VERSION} https://github.com/ppy/osu \