Ensure the docker image is based on geodata/gdal rather than homme/gdal

This reflects the change in repository name.

Ensure the docker image is based on geodata/gdal rather than homme/gdal

This reflects the change in repository name.

Ensure the docker image is based on geodata/gdal rather than homme/gdal

This reflects the change in repository name.

Updated version number to v0.2.2

* Ensure that the tile SRS is set if no reprojection is performed
* Warping is now multi-threaded using all CPU cores

Updated version number to v0.2.1

* Fix terrain height data type.  This should enable terrains at higher
  elevations to be created properly.

Updated version number to v0.2.0

* Enable GDAL supported raster tiles to be created
* Add support for the Global Mercator TMS profile
* Zoom levels range for processing can now be selected on the command
  line
* Add a Docker image
* Improve precision of `terrain-extents` GeoJSON output
* Various bug fixes

README fix