Hi,
You can use grafana-cli to change the admin password (in versions >4.1). To do this you need to run the command inside of the container using docker exec ....
Docs on resetting your password: http://docs.grafana.org/administration/cli/#reset-admin-password.
Example:
# run docker ps and find out the id of your grafana container
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
46590b4704b6 grafana/grafana "/run.sh" About an hour ago Up About an hour 0.0.0.0:3000->3000/tcp reverent_mcclintock
# switch out the id below for the one you got for your container
docker exec -ti 46590b4704b6 grafana-cli admin reset-admin-password --homepath /usr/share/grafana <your new admin password>