Wow. What a childish, rude, and condescending “welcome” to these forums.
Don’t worry, I’d already found the answer on my own:
The actual answer is that in the Docker, the files are not exposed to the host filesystem, only /config/ is. Therefore, /system/ was not present where I was looking (and because I’m new to Grav). Which means /system/ was in the actual Docker container.
You’re the one who needs to be excused, I’m afraid. I was just looking for help, not an insult.
Yikes.