docker/r10k/Dockerfile in r10k-3.5.2 vs docker/r10k/Dockerfile in r10k-3.6.0

@@ -1,15 +1,16 @@
-FROM alpine:3.9 as build
+ARG alpine_version=3.9
+FROM alpine:${alpine_version} as build
 
 # hadolint ignore=DL3018
 RUN apk add --no-cache ruby git && \
     mkdir /workspace
 WORKDIR /workspace
 COPY . /workspace
 RUN gem build r10k.gemspec && \
     mv r10k*.gem r10k.gem
 
-FROM alpine:3.9
+FROM alpine:${alpine_version}
 
 ARG vcs_ref
 ARG build_date
 ARG version="3.1.0"
 # Used by entrypoint to submit metrics to Google Analytics.