Improve docker container behavior when moving/copying a Confluence instance

Currently, when installing Confluence using Docker, it will start a new installation from scratch, which is not desirable in cases on which the end goal is to move an existing installation (or clone a Confluence instance, to create a test instance).

It would be useful to provide an option to start the Docker container with an already finished installation (with all of the settings already present on the existing confluence.cfg.xml file), as it would simplify this process.