Docker revival (Mode A): build in container, deploy to f3s

Multi-stage Dockerfile (Rocky 9 builder + runtime) builds ychat entirely in
a container with SSL/MySQL/readline disabled (in-memory guest chat). Three
minimal legacy-C++ patches so it builds on GCC 11:

- glob.h/modl.cpp: rename project 'function' typedef to 'mod_func_t'
  (collides with std::function via 'using namespace std')
- logd.cpp: 'ofstream == NULL' -> '!is_open()'
- sock.cpp: assign i_server_sock before set_nonblock() (was EBADF on startup)

Add DOCKER.md with build/push/deploy workflow and quirks (HTTP/0.9 responses,
ephemeral in-memory state).

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