Make the geoserver container populate the datadir, at runtime

[giohappy]

This task want to solve two issues at the same time:

1. get rid of the geoserver_data image and container, whose single purpose is to bake the content of the datadir volume
2. populate the datadir content at runtime instead of build time. This is required to allow bind mounting the datadirectory, instead of relying on Docker's internal volume. Explanation: the VOLUME directive inside the geoserver_data dir + the copying of content at build time forces the creation of an internal volume (that is shared with the geoserver image). If a bind mount is configured inside the Docker Compose configuration, it will be empty, since inside the container it's shadowed by the Docker volume. By moving the copy of datadir content at runtime we ensure that either the internal volume or a bind mounted volume is populated the first time the container is run.