How to create a secret docker secret?

You could try

printf $line | sudo docker secret create MYSQL_ROOT_PASSWORD -

and then

docker run --name some-mysql -e MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mysql-root -d mariadb:tag

The information concerning using secrets with MariaDB can be found on the MariaDB page of DockerHub.

"Docker SecretsAs an alternative to passing sensitive information via environment variables, _FILE may be appended to the previously listed environment variables, causing the initialization script to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in /run/secrets/<secret_name> files. For example:

$ docker run --name some-mysql -e MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mysql-root -d mariadb:tag

Currently, this is only supported for MYSQL_ROOT_PASSWORD, MYSQL_ROOT_HOST, MYSQL_DATABASE, MYSQL_USER, and MYSQL_PASSWORD"

You can use openssl rand option to generate a random string and pass to docker secret command i.e

openssl rand -base64 10| docker secret create my_sec -

The openssl rand option will generate 10 byte base64 encoded random string.