Support additional DockerRequirements where possible

[dleehr]

This method assumes a DockerRequirement with dockerPull, and will fail otherwise:

calrissian/calrissian/job.py

Lines 216 to 220 in b4526af

	def _get_container_image(self):
	# we only use dockerPull here
	# Could possibly make an API call to kubernetes to check for the image there, but that's not important right now
	(docker_req, docker_is_req) = self.get_requirement("DockerRequirement")
	return str(docker_req["dockerPull"])

Related to #7, which would inject a DockerRequirement prior to this call

[dleehr]

List of CWL DockerRequirements: https://www.commonwl.org/v1.0/CommandLineTool.html#DockerRequirement

Currently we only need/use dockerPull, but the others should be supported where possible

[dleehr]

dockerOutputDirectory is one of these requirements, closing #10 to consider it here.

[dleehr]

Also of note, #35 implements checking of the specified docker requirements and fails if they are not supported.