docker-compose: make nginx have public/build & other stuff independent from php container

[psineur]

TLDR: if possible - nginx & php should be able to live on any host, together or separately.
I think for prod-builds we can move package.json, npm install, npm run build-prod & dependencies of that onto nginx container, without using volumes and let it work this way.

This is probably tightly coupled with #192 - need to investigate how exactly are they being served.
IIRC they are going through PHP...

Blocked by #192