feat: Add codejail service

[timmc-edx]

This is just a skeleton image and doesn't have any directories or packages that are specific to codejail yet.

Lots of differences from cookiecutter django-ida default:

• Remove MySQL system packages
• Use latest Ubuntu LTS
• Shorten the base folder from codejail-service to codejail
• Use port 8080; we'll remap it in compose anyhow
• Use ADD on a git repo rather than RUN/curl/tar, which runs afoul of Docker caching when a branch is used.
• Allow selecting a commit version
• Pass DJANGO_SETTINGS_MODULE from outside as env var to reduce differences between dev and prod targets and add flexibility.
• Install in /app and create virtualenv in /venv; no need to hew to the same pattern we used in the configuration repo.
• Remove apt cache in same RUN command so as not to create a fat layer
• Use heredoc syntax for multi-command RUN
• No log files, just log to stdout

See edx/edx-arch-experiments#894

[timmc-edx]

I've been testing this with docker build --file dockerfiles/codejail.Dockerfile --build-arg VERSION=timmc/init --target=dev --tag codejail:timmc-dev-latest .

[robrap]

I'm doing a very cursory review, because I can't say I understand exactly what is happening and why. I'm approving, but let me know if you need me to be careful with any parts, or if you want me to test this.

I know we can iterate, so it doesn't seem like a big deal to just move ahead.