Fix Docker builds

[gabcoh]

Right now the dockerfile setup is pretty annoying for a few reasons:

1. It relies on caching build stages for the build not to take forever. This works, but I'd rather not rely on it. Better if the caching were a bit more explicit and maybe a bit more extensive
2. It only builds from git, so you can't really test things locally without pushing them. That is pretty horrible

[gabcoh]

Look into practices described here: https://docs.docker.com/develop/develop-images/multistage-build/

[cdecompilador]

The fact that docker is the main way to build this means that something is very wrong in the build-system files