Classroom: document use as autograder base image

[cuihtlauac]

Parent issue: #24

Finding (from UCSD study, Valstar et al., ITiCSE 2020)

"The DevContainer's Docker configuration can be used almost without change on auto grading systems such as Gradescope. This ensures that there will be no 'it works on my machine' issues with the auto grader."

This is a strong selling point: the same image students develop in is the image that grades their work. No environment mismatch is possible.

Proposed changes

• Document how to use ocaml-devcontainer as a Gradescope or similar autograder base image
• Provide an example Dockerfile for an autograder that extends the dev image:

  FROM ghcr.io/tarides/ocaml-devcontainer:latest
  COPY grading-scripts/ /autograder/

• Note that both switches (5.4.0 and 5.4.0+tsan) are available, so autograders can test student code under TSan for race conditions
• List compatible autograding platforms (Gradescope, GitHub Classroom autograding action)