The not very secure way (because docker inspect will show it) is to pass an environment variable to

docker run

such as

docker run -e password=abc

or in a file

docker run --env-file myfile

where myfile can contain

password1=abc password2=def

it is also possible to put them in a volume

docker run -v $(pwd)/my-secret-file:/secret-file

some better ways, use

keywhiz https://square.github.io/keywhiz/

vault https://www.hashicorp.com/blog/vault.html

etcd with crypt https://xordataexchange.github.io/crypt/