Tests failing in main due to dependence on deprecated `ContainerConfig`

[yuvipanda]

We are looking at ContainerConfig (added in #421) in our code. However, this option was deprecated by Docker (https://docs.docker.com/engine/deprecated/#container-and-containerconfig-fields-in-image-inspect) and removed.

This is now causing our tests to fail on main.

[yuvipanda]

It's really only used to get WorkingDir out. We should find a different, more explicit way to do that.