Is it possible to mount an s3 bucket as a point in a docker container?
No you can't. 

S3 is an object storage, accessed over HTTP or REST for example. Just as you can't mount an HTTP address as a directory you can't mount a S3 bucket as a directory.

Having said that there are some workarounds that expose S3 as a filesystem - e.g. 's3fs' project. How reliable and stable they are I don't know.