updates Dockerfile with new script source

Author: David O'Sullivan

ci/Dockerfile

@@ -1,7 +1,11 @@
 ARG base_image=openjdk:8-jre
+ARG script_image=springio/concourse-release-scripts:0.3.4
+
+FROM ${script_image}
+
 FROM ${base_image}
 
-ADD https://repo.spring.io/libs-release/io/spring/concourse/releasescripts/concourse-release-scripts/0.3.2/concourse-release-scripts-0.3.2.jar /opt/concourse-release-scripts.jar
+COPY --from=0 /concourse-release-scripts.jar /opt/
 
 RUN apt-get update && apt-get install --no-install-recommends -y \
     ca-certificates \
@@ -11,4 +15,4 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     && rm -rf /var/lib/apt/lists/*
 
 RUN curl -fL https://getcli.jfrog.io | sh && \
-    mv jfrog /usr/local/bin/
+    mv jfrog /usr/local/bin/