defining dockerFile without dockerImageId gives confusing error

[gijzelaerr]

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.

[mr-c]

See also #26