Problems with --cachedir and DockerRequirement

Hi,

the `--cachedir` option did not work for me (similar to #173).

I had a closer look and noticed that there is a difference in the `keydictstr` where it specifies the `"DockerRequirement":`.

In the one it puts an extra key `"dockerImageID":`, in the other one it doesn't:

```
..."DockerRequirement":{"class":"DockerRequirement","dockerPull":"GreetDB/pipeline:cwl_pipeline"},...
..."DockerRequirement":{"class":"DockerRequirement","dockerImageId":"GreetDB/pipeline:cwl_pipeline","dockerPull":"GreetDB/pipeline:cwl_pipeline"},...
```

As a temporary solution, I'm both specifying the `DockerImageId:` and the `DockerPull:` under the `DockerRequirement:`. The --cache-dir seems to be working consistently.

Thanks a lot,
Greet