Mount dir inside container same as outside + have work directory.

The config file outside the container uses path which resolve on the host machine. Inside the container this path needs to mapped to `/data/input`. It would be nice if inside the container the same paths can be used.

Something like
```python
client = BmiClientSingularity(image, input_dir='/shared/wflow/model-parameters', work_dir='/tmp/wflow-2000-2010')
client.initialize('wflow_sbm.ini')
client.update_until(client.get_end_time())
del client
```

Inside the container you would have
- `/shared/wflow/model-parameters` is mounted as `/shared/wflow/model-parameters` and `/tmp/wflow-2000-2010` is mounted as '/tmp/wflow-2000-2010'
- `/shared/wflow/model-parameters` directory with big parameter set for model
- '/tmp/wflow-2000-2010/wflow_sbm.ini config file for run

After model has run outside container you could have model output like logs for example `/tmp/wflow-2000-2010/wflow.log`.

Pros:
- inside/outside is same so less confusion
- use work_dir='.' to use same current working directory inside and outside container

Cons:
- paths in config file depend on machine, while before they could always use `/data/input`