Use a proper init for Docker images

@borsboom [wrote](https://github.com/commercialhaskell/stack/pull/2474#issuecomment-239212520):

> for the Docker case, maybe we should go back to having a separate 'init' process. We used to use phusion/baseimage's my_init in the containers but eliminated that for faster startup and compatibility with non-FP Complete-generated images. At that time we considered whether we should introduce our own extra 'init' process but decided to try without and see how it went.

I agree. IMHO Docker images should always use a proper init process. Even if you run GHC directly:
- signal handling will be different without an init process (signals without handler will be ignored)
- zombie processes won't be reaped
- more stuff might go wrong.

Minimal init binaries include dumb-init (used in the past by @snoyberg) and tini (used by me). I'll favor the smallest/most portable/possibly statically linked one.
#2474 was about a container launched manually, but IIUC the issue is still there for Stack Docker containers (to verify).