make docker image smaller

Dockerfile

@@ -1,14 +1,35 @@
-FROM ruby:2.7
+FROM ruby:2.7-alpine AS build-env
+
+WORKDIR /usr/src/app
+
+# Upgrade
+RUN apk update --no-cache
+
+COPY Gemfile Gemfile.lock ./
+RUN bundle config --global frozen 1 \
+    && bundle config --local deployment true \
+    && bundle config --local path vendor/bundle \
+    && bundle config --local without development \
+    && bundle config --local jobs 5 \
+    && bundle install \
+    && rm -rf vendor/bundle/ruby/2.7.0/cache/*.gem \
+    && find vendor/bundle/ruby/2.7.0/gems/ -name "*.c" -delete \
+    && find vendor/bundle/ruby/2.7.0/gems/ -name "*.o" -delete
+
+COPY amber-electric.rb .
+
+# Main image
+FROM ruby:2.7-alpine
 
 ENV INFLUXDB_HOSTNAME influxdb
 ENV INFLUXDB_DATABASE amber_electric
 
 WORKDIR /usr/src/app
 
-RUN bundle config --global frozen 1
-COPY Gemfile Gemfile.lock ./
-RUN bundle install
+# Upgrade
+RUN apk update --no-cache
 
-COPY . .
+COPY --from=build-env /usr/src/app /usr/src/app
+COPY --from=build-env /usr/local/bundle/config /usr/local/bundle/config
 
-CMD ["./amber-electric.rb"]
+CMD ["bundle", "exec", "./amber-electric.rb"]

Gemfile

@@ -4,10 +4,10 @@ source 'https://rubygems.org'
 
 git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 
+gem 'amazing_print'
 gem 'httparty'
 gem 'influxdb'
 
 group :development do
-  gem 'amazing_print'
   gem 'rubocop'
 end