[FEATURE] Extend section Dockerfile

[dwolf42]

In this section, you talk about creating the Dockerfile, which is absolutely fine for the scenario of crating the catnip image. But I don't feel confident creating such an image on my own, since I don't know what we wrote into that file and why. There is also the requirements.txt file, which you do not cover in this section, but it seems to play an important role when it comes to build our own images.

My suggestion is to elaborate more on the details of what each line of the Dockerfile and requirements.txt does, so a learner will be able to create an image from a Java cli program or something else.