/sbin/tini ends up running the Sass compiler, which seems to need a
larger thread stack size than the musl default.  This patch incorporates
a fix from
https://raw.githubusercontent.com/jubel-han/dockerfiles/master/common/stack-fix.c,
as described in https://github.com/sass/node-sass/issues/2031.

- improve docker_entrypoint.sh
- serve static files with puma by default
- sort packages list
- use virtual package for build deps
- show how to assign UID/GID