Check that docker images are installed before trying to launch containers

[JoshRosen]

Our tests should check that the required Docker images are installed before attempting to launch containers that use those images. I believe that Docker will automatically install missing images from the main docker.io repository, but it's confusing when this happens in a subprocess call since the output isn't printed to the console.

We should modify the launch container function in DockerUtils to check whether the image is installed and print a warning / error if it's missing rather than attempting to automatically build it.

We should also check for the images' presence in the init.sh environment setup script.