Possible workdir issue

[mikeh2]

I come without a lot of docker experience. But in the file sandboxes.py, the run function has a named parameter workdir. This is passed down the chain all the way to _create_sandbox_container

When that function (_create_sandbox_container) creates a container, it uses config.DOCKER_WORKDIR and NOT the incoming parameter.

Perhaps in run, the default value for workdir should be set to config.DOCKER_WORKDIR instead and pass that value down the chain.