refactor(server): colocate local driver config construction tests

[drew]

Description

Refactor the server-side local compute driver config construction so Docker and Podman config helpers and their tests live in a consistent module/location.

Context

PR #1785 review noted that docker_config_reads_bind_mount_opt_in_from_driver_table lives in crates/openshell-server/src/cli.rs, while the equivalent Podman test lives in crates/openshell-server/src/lib.rs.

That placement is currently explained by implementation locality: Docker uses the private build_docker_config(...) helper in cli.rs, and Podman uses podman_config_from_file(...) in lib.rs. The asymmetry is not a behavior bug, but it makes the config parsing tests harder to discover and compare.

Definition of Done

• Move Docker config construction out of cli.rs into a server config module or another shared location that matches the Podman path.
• Move docker_config_reads_bind_mount_opt_in_from_driver_table with the Docker config constructor.
• Keep CLI/config behavior unchanged.
• Preserve or improve coverage for Docker and Podman driver-table parsing and config merge precedence.
• Update references if any docs or comments name the old helper location.