defining dockerFile without dockerImageId gives confusing error

When I define a CWL definition containing a dockerFile statement without dockerImageId I get:
```
$ .virtualenv/bin/cwltool 1.0.20171017195544
Resolved 'steps/ampl.cwl' to 'file:///home/gijs/prefactor-cwl/steps/ampl.cwl'
Workflow error, try again with --debug for more information:
Docker is not available for this tool, try --no-container to disable Docker, or install a user space Docker replacement
like uDocker with --user-space-docker-cmd.: 'dockerImageId'
```

When I run the command with --debug enabled I see the error:

```
KeyError: 'dockerImageId'
```

after adding a docker image ID the problems goes away.

Reproduce the CWL definition with a dockerFile but no dockerImageId, first without and then with --debug, using the command shown in the issue. Trace where the missing dockerImageId becomes a KeyError; done means the input produces a clear validation error that identifies the missing field instead of the misleading Docker availability message.

Written by the indexing model from the issue text.