Add build_mingw target using x86_64-w64-mingw32-gcc cross-compiler with
dedicated MinGW flags and publish step. Introduce Dockerfile with Ubuntu
base image installing all required build dependencies (gcc, make, readline,
ncurses, curl, openssl, json-c, sqlite3, python3-dev). Add compose.yml for
convenient Docker-based development shell with mounted working directory.
Extend Makefile with docker, docker_make, docker_run, run_mingw targets and
suppress error output on publish commands for build and build_free targets.
