After you log in to your private image registry with the Docker login command, a warning is displayed that indicates that your password is stored unencrypted.
Causes
By default, Docker stores the login password unencrypted within the /root/.docker/config.json Docker configuration file. This is the default Docker behavior.
Resolving the problem
You can store your user credentials in an external credentials store instead of within the Docker configuration file. Storing your credentials in a credentials store is more secure than storing the credentials in the Docker configuration file. For more information.
According to docker documentation:
To run the docker login command non-interactively, you can set the --password-stdin flag to provide a password through STDIN. Using STDIN prevents the password from ending up in the shell’s history, or log-files.
The following examples read a password from a file, and passes it to the docker login command using STDIN:
$ cat ~/my_password.txt | docker login --username foo --password-stdin
OR
$ docker login --username foo --password-stdin < ~/my_password.
The following example reads a password from a variable, and passes it to the docker login command using STDIN:
$ echo "$MY_PASSWORD" | docker login --username foo --password-stdin
Reference: Docker: Using --password via the CLI is insecure. Use --password-stdin
https://www.ibm.com/docs/en/cloud-private/3.2.0?topic=login-docker-results-in-unencrypted-password-warning