Normalize container type case

Running `client.register_container` expects for `docker` for `container_type`, the endpoint `Config` class expects `Docker`. We should normalize these to one or the other throughout the platform, or, as @ZhuozhaoLi suggests, make it case-insensitive.

Start by tracing client.register_container and the endpoint Config class to compare how container_type values are handled. Resolve the stated case mismatch across the platform, then verify that registration and endpoint configuration use one consistent or case-insensitive convention.

Written by the indexing model from the issue text.