Docker container is not updated/rebuild on definition change

When a container is created by cwltool using a definition like this it works:
```
hints:
DockerRequirement:
dockerImageId: kernsuite/prefactor
dockerFile: |
FROM kernsuite/base:3
RUN docker-apt-install prefactor
```

But when I change/update the dockerFile statement the old docker container is used, the content is not updated or rebuild.

Start by reproducing the issue with the supplied CWL DockerRequirement, then change the dockerFile and observe whether cwltool reuses the old container. Done means a changed dockerFile causes the container image to be updated or rebuilt rather than using the previous content.

Written by the indexing model from the issue text.