Add option to specify another base image

[hussainweb]

It would be good to be able to specify this option in composer.json. We don't need this option to be overridden on the CLI as it is not likely to change often.

If possible, we should validate that the image would support importing databases the way we do right now. Since that may not be feasible, we could consider showing a warning if the base image is not from Bitnami or other known sources (we could find other supportable base images in the future).