Add a way to update dotrun-image

Currently there doesn't seem to be a way to update dotrun-image. Once you have dotrun-image downloaded by docker dotrun will just run without checking if new version is released.

There is no way of knowing that new version is released and no way to check or update via docker itself.

You have to know that new image was pushed to docker, and either remove docker-image locally or pull new one manually from docker.

## Expected

`dotrun` command should notify users when new version of dotrun (and dotrun-image) is available and give an option to upgrade.