Documentation improvement regarding image building options

[edudar]

Extracted from #84.

I think it might help to define options that developers have from the start as in Table of Content style. Something in the lines of "You can build nice and shiny spring boot image in the following ways: * dockerfile, * buildpacks using maven/gradle, * jib using its own plugins, * ..." and then expand each bullet point/option with more details. Because what may happen now, is that the developer goes through a tutorial that starts with a lengthy explanation of how to setup Dockerfile properly. They do it step by step. They end up with said Dockerfile and then, if careful, they might see that they didn't actually have to setup Dockerfile manually because there are other options. Or, as happened in my team, they will use bootBuildImage and Dockerfile together.