Some minor improvements for setup script

Since we use `yarn` for our container setup and try to enforce
dependencies, we should also use yarn in the setup script.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

bin/setup

@@ -8,11 +8,12 @@ if [ -d .git ]; then
   cd "$(git rev-parse --show-toplevel)"
 fi
 
-if ! type npm > /dev/null
+if ! type yarn > /dev/null
 then
   cat << EOF
-npm is not installed, please install Node.js and npm.
+yarn is not installed, please install Node.js, npm and yarn.
 Read more on Node.js official website: https://nodejs.org
+And for yarn package manager at: https://yarnpkg.com/en/
 Setup will not be run
 EOF
   exit 0
@@ -27,8 +28,9 @@ if [ ! -f .sequelizerc ]; then
   cp .sequelizerc.example .sequelizerc
 fi
 
-echo "install npm packages"
-BUILD_ASSETS=false npm install
+echo "install packages"
+yarn install --pure-lockfile
+yarn install --production=false --pure-lockfile
 
 cat << EOF