update apt only once

Dockerfile

@@ -1,8 +1,7 @@
 FROM ruby:2.6
-RUN apt-get update -qq && apt-get install -y nodejs postgresql-client
 RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
 RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
-RUN apt update && apt install yarn
+RUN apt-get update -qq && apt-get install -y nodejs postgresql-client yarn
 RUN mkdir /myapp
 WORKDIR /myapp
 COPY Gemfile /myapp/Gemfile