fix: Use the SAM build images to build image based Ruby functions (#503)

* Install dev tools before running bundle in image

* The command is remove, not uninstall

* Use build images instead of installing tools in runtime environment

* Remove duplicate comment

ruby/hello-img/{{cookiecutter.project_name}}/hello_world/Dockerfile

@@ -1,9 +1,23 @@
-FROM public.ecr.aws/lambda/ruby:{{ cookiecutter.options[cookiecutter.runtime].version }}
+# building phase
+#
+FROM public.ecr.aws/sam/build-ruby{{ cookiecutter.options[cookiecutter.runtime].version }} as build-image
+
+ARG SCRATCH_DIR=/var/task/build
+
+ENV GEM_HOME=${SCRATCH_DIR}
 
 COPY app.rb Gemfile ./
+RUN bundle install
+RUN cp -r ${GEM_HOME}/* .
+RUN rm -r ${GEM_HOME}
+
+# runtime emulation phase
+#
+FROM public.ecr.aws/lambda/ruby:{{ cookiecutter.options[cookiecutter.runtime].version }}
 
 ENV GEM_HOME=${LAMBDA_TASK_ROOT}
-RUN bundle install
+
+COPY --from=build-image /var/task/ ./
 
 # Command can be overwritten by providing a different command in the template directly.
 CMD ["app.lambda_handler"]