Investigate if splitting build steps into multiple build steps improves reusebility of the images

[tkilias]

Background

• In (Add option to) Re-use images from a previous build run #245 was asked if we keep the docker build cache and if we can use it, that might be difficult
• However, exaslct already provides a mechanism for reusing images, but in certain cases it might be too coarse.
• Splitting up some build steps into multiple, might improve the reuse of images and with this, build speed.
• A example could be, if we install apt packages, python packages and r packages in the same build step, we could split this into three build steps. We could put them in on Pipeline or use a diamond pattern, where we first install the apt packages, have then two build steps in parallel which install the python and r-packages and at last copy the updates into one images.