[UX] Avoid rebuilding the project

installing the project with pip install -e . goes through a build phase and needs to download and install build dependencies.
Sometimes this is not needed if the project is just Python code, or if it can be cashed.
How can this be avoided? It creates a delay at the start of every container.

#27 adds the build dependencies in the images and builds without isolation. This makes the build much faster in the cases where it's only Python code to link.