Update dockerfile to create 90% smaller image

Dockerfile

@@ -1,6 +1,21 @@
-FROM ruby:2.4
+# Phase 1: Build the gem and all dependencies, needs C compiler
+FROM ruby:2.4-alpine as builder
 
-RUN gem install redis-stat
+RUN apk add --no-cache build-base
+
+ENV GEM_HOME "/gems"
+
+RUN gem install -N redis-stat
+
+# Phase 2: Build the actual docker image, needs no C compiler -> much smaller image
+FROM ruby:2.4-alpine
+
+RUN apk add --no-cache ncurses
+
+ENV GEM_HOME "/gems"
+ENV PATH $GEM_HOME/bin:$GEM_HOME/gems/bin:$PATH
+
+COPY --from=builder /gems /gems
 
 EXPOSE 63790
 ENTRYPOINT ["redis-stat"]