* SDK - Containers - Added support for container image cache
This change makes `build_image_from_working_dir` fast when the working directory has not changed between invocations.
We cache pushed container images using specially-calculated context directory hash as the cache key.
* Moved the import to the top
* SDK - Containers - Build container image from current environment
* Removed the ability to capture the active python environment (as requested by @hongye-sun)
* Added the type hint and docstring to for the return type.
* Renamed `build_image_from_env` function to `build_image_from_working_dir`
as requested by @hongye-sun
* Explained the function behavior in the documentation.
* Removed extra empty line
* Improved caching by copying python files only after installing python packages
* Made test more portable
* Added support for specifying the base_image
`kfp.containers.default_base_image = ...`
The image can also be a callable returning the image name.
* Renamed `get_python_image` to `get_python_image_for_current_version`
* Switched the default base image to Google Deep Learning container image as requested by @hongye-sun
The size of this image is 4.35GB which really concerns me. The GPU image size is 6.45GB.
* Stopped importing kfp.containers.* into kfp.*
* Fixed test
* Fixed the regex string
* Fixed the type annotation style
* Addressed @hongye-sun feedback
* Removed the container image size warning
* Fixed import failure
Alexey Volkov