Custom docker images

[williams-jack]

Feature/Problem Description

Orca "worker" machines (i.e., machines running the grading-vm logic) should be able to support the use of custom Docker images, such that a professor can ensure an environment that has all the PL support and utilities needed to properly grade an assignment.

NOTE: This PR only supports custom image logic on the VM side. Logic on the Orca web server side will be added in a later PR.

Solution (Changes Made)

• Ability to download docker image files (in the form of .tgz archives).
• Utilities for cleaning up unused docker images.
• CLI support for devs to run basic examples.
• Exception handling for containers that time out.
• Proper file handling for consistency across compression/archive types.
• Ability for nested grading scripts.

[williams-jack]

~Please don't let students have root access inside the containers.~

Resolved.

[williams-jack]

~Still missing in this PR: test case where we confirm downloading an image and loading it works properly~ Resolved.