Add `WorkingDir` field when creating containers

[inohmonton99]

Add `WorkingDir` field when creating containers

[terrytangyuan]

Per our discussion offline, this is a request to support workingDir at container spec level, e.g. :

- container:
    args:
    - |
      apt-get update \
      && apt-get install -y git wget libglib2.0-0 libsm6 libxext6 libxrender-dev \
      && pip install -r requirements.txt
      && cd /mnt/src/ \
      && python ...
    command:
    - sh
    - -c
    image: xxx
    workingDir: /mnt/src