Add pip in the step image. (#2866)
Add pip in the step image. (#2866)

* Make the attribute check for XGBoost model compatible with reg:linear

* Add pip in the step image. Because we will pip install additional python packages while building runnable image based on step image.

* Install python3-dev in the builder stage.
sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow