Fix yarn dependency

Author: falusi94

Dockerfile

@@ -1,6 +1,12 @@
 FROM ruby:2.3
 RUN apt-get update -qq &&\
-    apt-get install -y build-essential libpq-dev nodejs libssl1.0-dev imagemagick
+    apt-get install -y build-essential libpq-dev nodejs libssl1.0-dev imagemagick apt-transport-https
+
+RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - &&\
+    echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list &&\
+    apt-get update && apt-get install yarn
+
+RUN export PATH="$PATH:/opt/yarn-[version]/bin"
 
 RUN mkdir /leltar
 WORKDIR /letar