chore(deps): update nginx docker tag to v1.21.6

Author: renovate-bot

client/Dockerfile

@@ -9,7 +9,7 @@ COPY . /app
 RUN npm run build
 
 # production environment
-FROM nginx:1.16.0-alpine
+FROM nginx:1.21.6-alpine
 COPY --from=build /app/build /var/www
 COPY nginx/nginx.conf /etc/nginx/nginx.conf
 EXPOSE 80