Update yarn in Dockerfile to 1.22.21

docker/Dockerfile

@@ -9,7 +9,7 @@ COPY package.json babel.config.json webpack.config.js yarn.lock /app/
 # ensure we have python-venv available for glean
 RUN apt-get update && apt-get install python3-venv -y
 
-RUN npm install -g --force yarn@1.22.19
+RUN npm install -g --force yarn@1.22.21
 RUN yarn install
 RUN yarn build