Add support for S3 mounts in the Docker runtime

[runabol]

Mountpoint S3 is a file client that translates local file system API calls to S3 object API calls like GET and LIST.

The goal of this feature is to add first-class support to mounting S3-based location so users don't have to rely on using the AWS S3 client and have a more streamlined experience that resembles mounting a local directory.

Rough outline of the implementation:

1. Add an s3.go file to the docker packge.
2. Implement the Mounter interface.
3. Register the new mounter implementation when wiring in the Docker runtime.
4. Add tests.