[discourse-web] Install yarn too (npm is pulled in by nodejs package)

[Bartłomiej Piotrowski <bpiotrowski src gnome org>]

commit 3382691ecca0ea04f3a47cf17e12d56ac09ecf25
Author: Bartłomiej Piotrowski <bpiotrowski gnome org>
Date:   Mon Feb 17 13:25:23 2020 +0100

    Install yarn too (npm is pulled in by nodejs package)

 Dockerfile | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/Dockerfile b/Dockerfile
index 8b46a88..1c51334 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -7,14 +7,16 @@ RUN apt update && \
     echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > 
/etc/apt/sources.list.d/pgdg.list && \
     wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add - && \
     curl -sL https://deb.nodesource.com/setup_10.x | bash - && \
+    echo "deb https://dl.yarnpkg.com/debian/ stable main" > /etc/apt/sources.list.d/yarn.list && \
+    curl -sL https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -  && \
     apt-get update && apt-get -y upgrade && \
     apt-get -y install build-essential gettext \
     # Install postgres
     postgresql-client-10 \
     # Install image utilities
     advancecomp jpegoptim gifsicle libjpeg-turbo-progs libpng-dev optipng pngcrush pngquant jhead gsfonts \
-    # Install nodejs and npm
-    nodejs npm \
+    # Install nodejs and yarn
+    nodejs yarn \
     # Needed by init-assets.sh
     brotli && \
     npm install -g uglify-js && \