Learner SQL piped into the mysql client could run `\! id` as container
root and read every DB. Now: --binary-mode (client commands error out),
a refused-backslash-line guard, learner exec as uid 65534, and a random
root password (0600 file, via env, adopted live on a legacy container).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WzxdWYGW5NZL5uVpnqmNKo
