Reduce the size of kernel docker images

[achimnol]

In Sorna REPL repository and its Docker hub counterpart, we maintain several container images for different programming environment.

Currently, most images are over 400 MB and often 1~2 GB in size. This prevents fast iteration of testing and deployments. Let's reduce the image size using various techniques, such as sharing a common base image, building from smaller base images, using prebuilt system and binary packages, and/or removing unncessary packages.

Another approach is to use rocker so that build processes can utilize shared pip cache directories and etc.

[achimnol]

This issue has no longer valid context.
Our focus is to provide "working" images not "small" images, though technically we should favor smaller-sized images when possible, but we should not sacrifice the functionality and delivery time.