Minio file system fix

Following sleuthing by Alex and Vignesh, this fixes Minio file hosting errors (description here: https://github.com/minio/minio/issues/13025 ). The bug was that underscores in domains like file_hosting (which we were using, as is Editoria) don't work on some architectures, though it does work with others. This MR changes file_hosting to fileHosting in docker-compose.yml. It also changes this line in .env.EXAMPLE:

S3_HOST=fileHosting

which will probably need to be changed in your .env before this will work.

See #903