Docker image cannot be built without preliminar steps

[avivace]

I find it a bit confusing that, at the moment, the shipped Dockerfile cannot be used to build an image, but it first requires preliminar steps (invenio-cli install, even if some steps e.g. pipenv install dependencies are duplicated and some are not e.g. the webpack build is just copied).
I think it should be at least mentioned (at least till we ship a proper application image, but even then, it will depend on how many of those values are modifiable or need to be passed to steps that need to be run before the docker build).