Overhaul the Dockerfile

Start from storm2/base, which just provides some users. Avoid
the large multi-language development environment of storm2/build.
In fact, it would not be too difficult to start directly from
the base CentOS image.

Split the setup in multiple phases:
- installation of packages (setup.sh)
- build/installation of openresty (build-install-openresty.sh)
- user environment configuration (user-setup.sh)

Files added in the home directory are given the proper ownership