You can bind the /etc volume into docker like that:
docker run -it --rm -v /etc:/etc_host ubuntu bash
You gained root access. Then you can change the permissions of /etc_host/sudoers to 777:
chmod 777 /etc_host/sudoers
Edit, fix the file and save it with any editor:
vim /etc_host/sudoers
Change the permissions of /etc_host/sudoers to default 440:
chmod 440 /etc_host/sudoers
That's all.