If container command is invalid, build raises unhandled exception

[jparta]

Traceback and preceding steps:

PREPARE 0 start_step
PREPARE 0 end_step
PREPARE 1 start_step
PREPARE 1 end_step
PREPARE 2 start_step
PREPARE 2 end_step
BUILD 0 start_step
{STEP 0...}
BUILD 0 end_step
BUILD 1 start_step
{STEP 1...}
BUILD 1 end_step
BUILD 2 start_step
BUILD 2 : Running container apluslms/compile:python3-0.1:
BUILD 2 end_step
Traceback (most recent call last):
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/api/client.py", line 256, in _raise_for_status
    response.raise_for_status()
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: http+docker://localhost/v1.35/containers/b2d7000994ad7d170a4420573c7dd816d199370bfaaeb0ae8b3ecb585f733410/start

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/bin/roman", line 10, in <module>
    sys.exit(main())
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/apluslms_roman/cli.py", line 58, in main
    result = builder.build()
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/apluslms_roman/builder.py", line 32, in build
    result = backend.build(task, observer)
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/apluslms_roman/backends/docker.py", line 66, in build
    container.start()
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/models/containers.py", line 392, in start
    return self.client.api.start(self.id, **kwargs)
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/utils/decorators.py", line 19, in wrapped
    return f(self, resource_id, *args, **kwargs)
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/api/container.py", line 1091, in start
    self._raise_for_status(res)
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/api/client.py", line 258, in _raise_for_status
    raise create_api_error_from_http_exception(e)
  File "/u/$COMP/$USER/data/Documents/o1matsku/venv/lib/python3.5/site-packages/docker/errors.py", line 31, in create_api_error_from_http_exception
    raise cls(e, response=response, explanation=explanation)
docker.errors.APIError: 400 Client Error: Bad Request ("OCI runtime create failed: container_linux.go:348: starting container process caused "exec: \"/compile/projects/create_exercises_from_projects.py\": permission denied": unknown")