Install yarn in GDK docker image

Closes #221 (closed)

Dockerfile

@@ -22,7 +22,12 @@ RUN apt-get install -y git postgresql postgresql-contrib libpq-dev redis-server
   libreadline-dev npm sudo nodejs-legacy
 # GDK tools
-RUN apt-get install -y net-tools psmisc
+RUN apt-get install -y net-tools psmisc apt-transport-https
+
+# install yarn
+RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
+RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+RUN apt-get update && apt-get install -y yarn
 # Add GDK user
 RUN useradd --user-group --create-home gdk