Can't access to volume and got getcwd: No such file or directory

I have two sites on docker like below

and both of them has an own volume on Nginx container

      - ./site1/prod/static/:/var/www/site1/html
      - /home/my_user/docker/site2/wedding_card/priv/static:/var/www/site2/html

my problem is that after many hours one of the volumes inside Nginx container disconnected and static files not loaded

even I run this command

sudo docker exec -it nginx_master ash
cd /var/www/site1/html
ash: getcwd: No such file or directory

but cant access
after a restart, Nginx container problem is gone

I did search on google and find this

why I did get this problem?