Can't 'add' files to a docker container

currently this doesnt work:
```
requirements:
- class: DockerRequirement
dockerFile: |
FROM kernsuite/base:2
RUN docker-apt-install casalite
ADD casawrap.py /usr/local/bin
```
I'm trying to a add a file to the container. Probably we don't want to implicitly copy all kind of files to the tempfolder used for constructing the container, and it is probably als not a good idea to use the current work folder or the cwl base folder as a build directory for docker since then all files in those folders are copied to the docker engine. I think it maybe is a good idea to extent the `DockerRequirement` class with a for example`stage files` directive the copies a list of specified files to the Docker build work folder so they can copied to inside the container.