The nimiq-db-exporter docker image consists of two parts: the exporter itself and a MariaDB image as its database backend. These two parts are glued together with a docker-compose file. The docker-compose file as well as the respective Docker files can be found in misc/docker/nimiq-db-exporter.
The containers can be brought up by simply executing docker-compose up -d in the directory containing the docker-compose file. The command docker-compose down stops and removes the containers again. This includes all data in the container instances expect for the Nimiq blockchain and the MySQL database. These databases are saved persistently on seperate volumes managed by the docker daemon. These volumes can be configured in the top-level volume key in the docker-compose file. More information can be found in the docker-compose reference (see [1] and [2]).
The commands docker-compose stop and docker-compose start respectively temporarily terminate the container and restart it, however, without deleting the containers (in contrast to down and up).
On the first execution, the base images are pulled from the Docker registry and/or the images are built from the enclosed Dockerfiles. Additionally, the MySQL database is initialized.
By default, the MariaDB image exports the daemon on port 3306 to the host locally only. This port forwarding setting can be changed in the docker-compose file to make the port externally reachable. The database is nimiq with user nimiq and password nimiq. The root password is set to a random value during initialzation, which can be found in the logs. More information on the MariaDB docker image can be found here [3].
In theory, the containers (and images) could be updated by pulling any changes from this repository and executing docker-compose down && docker-compose up -d. However, as the images are not (most likely) yet available in the Docker registry, Docker is unable to pull newer images and its caching mechanism won't rebuild the images as well. Therefore, for the time being docker-compose should be called with docker-compose up -d --build which will then force a complete container/image rebuild.
One needs to occasionally check for image/container updates oneself, since this feature is not provided by the Docker daemon.
[1] https://docs.docker.com/compose/reference/