Docker Wordpress timeout limit error db connection - mysql

Question

For the first time I'm using Docker on Digital Ocean for a Wordpress installation. The setup is ok, also the theme installation, but when I install the Revolution Slider plugin the server show me: "Error establishing a database connection".
Plugin installed successfully.
Warning: mysqli_query(): MySQL server has gone away in /var/www/html/wp-includes/wp-db.php on line 1877
Warning: mysqli_query(): Error reading result set's header in /var/www/html/wp-includes/wp-db.php on line 1877
IMAGE COMMAND CREATED STATUS
mysql:latest "docker-entrypoint..." About an hour ago Restarting (1) 12 minutes ago
I restarted all the container, Wordpress works but I can't see any Plugin and the Theme page show me only "ERROR: The theme directory themename does not exist", but looking the FTP the directory is there. Any suggestion? Thanks!
docker-compose.yml
version: '2'
services:
digitalpmi:
container_name: digitalpmi_wp
depends_on:
- db
image: wordpress:latest
restart: always
volumes:
- ./wp-content:/var/www/html/wp-content
environment:
- VIRTUAL_HOST=[ip]
- WORDPRESS_DB_NAME=dpdb
- WORDPRESS_TABLE_PREFIX=dp_
- WORDPRESS_DB_HOST=db
- WORDPRESS_DB_PASSWORD=[password]
networks:
- front
- back
db:
container_name: digitalpmi_db
image: mysql:latest
restart: always
volumes:
- db_data:/var/lib/mysql
environment:
MYSQL_ROOT_PASSWORD:[password]
networks:
- back
phpmyadmin:
container_name: phpmyadmin
depends_on:
- db
image: phpmyadmin/phpmyadmin
restart: always
ports:
- 8181:80
environment:
PMA_HOST: db
MYSQL_ROOT_PASSWORD:[password]
networks:
- back
nginx-proxy:
image: jwilder/nginx-proxy
container_name: nginx-proxy
ports:
- "80:80"
- "443:443"
volumes:
- /var/run/docker.sock:/tmp/docker.sock:ro
networks:
- front
networks:
front:
back:
volumes:
db_data: