Enforce environment variable configuration for Docker entrypoint

- Updated `docker-entrypoint.sh` to disallow custom command-line arguments, requiring configuration through `SCRATCHBOX_*` environment variables.
- Modified `README.md` to clarify that custom container commands are not supported and to reflect the new entrypoint behavior.
- Enhanced `main_test.go` to validate failure when required environment variables are not set.
- Added a function in `server.go` to print startup environment variables, ensuring visibility of configuration during server initialization.

README.md

@@ -51,14 +51,11 @@ Use the included Compose example:
 Inside the container, the default command is:
 
 - `scratchbox server --env`
+- Custom container command/args are not supported; configure behavior via `SCRATCHBOX_*` env vars.
 - The image forces `SCRATCHBOX_STORAGE_DATA_DIR=/data`.
 - The image forces `SCRATCHBOX_SERVER_LISTEN_ADDR=:8080`.
 - The image forces `SCRATCHBOX_SERVER_ACCESS_LOG_FILE_PATH` to empty.
 
-To run with a YAML config instead, pass `--config` explicitly (mutually exclusive with `--env`):
-
-- `docker run --rm -p 8080:8080 -v "$(pwd)/docker-data:/data" -v "$(pwd)/docker-config:/config" scratchbox:dev server --config /config/config.yaml`
-
 Storage encryption key behavior:
 
 - The key file is always stored at `storage.data_dir/metadata.key` (next to `metadata`).