Modify step image build process (#2397)
Modify step image build process (#2397)

* Modify step image build process

* add copyleft

* polish step image build script

* remove choose_fastest_apt_pip_sources.sh
sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow