Traverse symlinked directories within tilesets directory

[brendan-ward]

Right now, it seems like we are not traversing tilesets that are in symlinked directories within the tileset directory.

This would allow symlinking multiple subdirectories into a single tileset directory provided to the server.

[asmyasnikov]

In mounted volume in docker container symlinks to out from volume may be broken =(

[brendan-ward]

@asmyasnikov is the same true when you provide multiple root directories to Docker, as per #92? I would expect that you'd need to mount host directories as volumes for your Docker container.

[asmyasnikov]

Yes, you right. I will mount each directory separately as volumes. But if feature of multiple paths you not approve, I must be use symlinks. Valid symlinks inside docker may be invalid outside. And vice versa.