gdalcubes_worker binary and Docker image for scalability

[appelmar]

To scale analysis in shared-nothing environments, a Docker image which runs a gdalcubes_worker binary that listens to incoming tasks would be useful.

[appelmar]

Now under development since appelmar/gdalcubes@60f1e37

[appelmar]

A first prototype is now working (see appelmar/gdalcubes@7813f96). So far, swarm clients are not fault tolerant.