singular-compose does not support a default command

A `_default_command` should probably be added to the singular-compose.env file format, so that. Right now there is no way to duplicate the `command:` line in a docker-compose file.

- This would not cover overriding the entrypoint, which is less important.
- This would only trigger if `${@}` was empty, for both `instance strart` and `run`