Chore: Build: Add build-time LICENSE_REQUIRED config flag to Joplin Server (#15913)

Dockerfile.server

@@ -89,6 +89,9 @@ RUN pm2 install https://registry.npmjs.org/pm2-logrotate/-/pm2-logrotate-3.0.0.t
 	&& pm2 set pm2-logrotate:retain 5 \
 	&& pm2 set pm2-logrotate:compress true
 
+ARG LICENSE_REQUIRED="0"
+ENV LICENSE_REQUIRED=${LICENSE_REQUIRED}
+
 ENV NODE_ENV=production
 ENV RUNNING_IN_DOCKER=1
 EXPOSE ${APP_PORT}