fix(compose): stop creating config.toml as a directory

[floze-the-genius]

What does this do?

Closes #258.

The provision wizard writes docker/config.toml as a regular file, but the Propeller Compose file used short bind-mount syntax for that path. If Compose is started before provisioning, Docker creates a missing bind source as a directory. The wizard then cannot replace that directory with the generated TOML file and reports failed to create docker/config.toml file.

This converts all three config mounts (manager, proplet, and proxy) to long bind syntax with bind.create_host_path: false.

Compose will now fail clearly when the provisioned config is missing instead of mutating the host path into a directory. Existing mount targets and runtime behavior are unchanged once the file exists.

Users who already have an empty docker/config.toml/ directory still need to remove it once before running the provision command; Compose will not recreate it afterward.

Verification

• Parsed docker/compose.propeller.yaml with Docker Compose v5.1.2.
• Inspected normalized Compose JSON and verified all 3 config mounts have bind.create_host_path == false.
• Ran go test ./... successfully.
• Ran git diff --check successfully.