Switch the webapp to a multi-stage build to reduce size and rebuilds

This should help avoid unnecessary incremental builds locally

webapp/Dockerfile

@@ -1,27 +1,39 @@
-FROM ruby:2.3-alpine
+FROM ruby:2.3-alpine as builder
 
-ARG BUILD_PKGS="ruby-dev openssl-dev gcc make libc-dev binutils"
 ARG APP_DIR=/webapp
-
-RUN mkdir -p ${APP_DIR}/vendor
-
-WORKDIR ${APP_DIR}
+ARG BUILD_PKGS="ruby-dev openssl-dev gcc make libc-dev binutils"
 
 ENV GEM_HOME=${APP_DIR}/vendor/gems
 ENV BUNDLE_PATH=${APP_DIR}/vendor/gems
 ENV BUNDLE_APP_CONFIG=${APP_DIR}/vendor/gems/.bundle
 ENV BUNDLE_DISABLE_SHARED_GEMS=true
 
+RUN mkdir -p ${APP_DIR}/vendor
+
+WORKDIR ${APP_DIR}
 ADD Gemfile* ${APP_DIR}/
+RUN apk add -U ${BUILD_PKGS} && \
+        bundle install && \
+        apk del ${BUILD_PKGS} && \
+        rm -rf /var/cache/apk/*
+
+# Doing a multi-stage build to reset some stuff for a smaller image
+FROM ruby:2.3-alpine
+
+ARG APP_DIR=/webapp
+WORKDIR ${APP_DIR}
+
+COPY --from=builder ${APP_DIR} .
+
+ENV GEM_HOME=${APP_DIR}/vendor/gems
+ENV BUNDLE_PATH=${APP_DIR}/vendor/gems
+ENV BUNDLE_APP_CONFIG=${APP_DIR}/vendor/gems/.bundle
+ENV BUNDLE_DISABLE_SHARED_GEMS=true
+
 ADD app.rb ${APP_DIR}/
 ADD monkeys.txt ${APP_DIR}/
 ADD config.ru ${APP_DIR}/
 ADD views ${APP_DIR}/views/
 ADD assets ${APP_DIR}/assets/
 
-RUN apk add -U ${BUILD_PKGS} && \
-        bundle install && \
-        apk del ${BUILD_PKGS} && \
-        rm -rf /var/cache/apk/*
-
 CMD bundle exec puma