Separate Dockerfile for Data and Application

This is based on #1158

Instead of creating data container from same docker image, we create separate docker image for data container to avoid keeping original docker image after upgrading to new version. (That results in 1GB space that cannot be freed)

See merge request !409

The problem is `docker stop` only sends SIGTERM to the PID 1 inside the
container, and the PID 1 (`/bin/sh -c ...`) does not take care of
signals. Hence the services (e.g., postgresql, redis, sidekiq, etc)
never have chances to graceful shutdown. Docker just kills the container
after its 10 seconds timeout by default.

What this commit does:

1) Add a wrapper as the default executable of Docker container. Which
   starts services through `runit`, reconfigure Gitlab by `gitlab-ctl`
   and gracefully shutdown all services when a SIGTERM is received.
2) Create an `assets` directory for assets.
3) Add `.dockerignore` file.

Now you'll see the following log messages after `docker stop`:
```
SIGTERM signal received, try to gracefully shutdown all services...
ok: down: logrotate: 1s, normally up
ok: down: nginx: 0s, normally up
ok: down: postgresql: 1s, normally up
ok: down: redis: 0s, normally up
ok: down: sidekiq: 0s, normally up
ok: down: unicorn: 0s, normally up
```

Signed-off-by: kfei <kfei@kfei.net>