Gracefully shutdown services in Docker container

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>

docker/.dockerignore

@@ -0,0 +1 @@
+*.md

docker/Dockerfile

@@ -26,9 +26,12 @@ RUN mkdir -p /opt/gitlab/sv/sshd/supervise \
 # Expose web & ssh
 EXPOSE 80 22
 
-# Volume & configuration
+# Declare volumes
 VOLUME ["/var/opt/gitlab", "/var/log/gitlab", "/etc/gitlab"]
-ADD gitlab.rb /etc/gitlab/
 
-# Default is to run runit & reconfigure
-CMD gitlab-ctl reconfigure & /opt/gitlab/embedded/bin/runsvdir-start
+# Copy assets
+COPY assets/gitlab.rb /etc/gitlab/
+COPY assets/wrapper /usr/local/bin/
+
+# Wrapper to handle signal, trigger runit and reconfigure GitLab
+CMD ["/usr/local/bin/wrapper"]

docker/assets/wrapper

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+function sigterm_handler() {
+    echo "SIGTERM signal received, try to gracefully shutdown all services..."
+    gitlab-ctl stop
+}
+
+trap "sigterm_handler; exit" TERM
+
+function entrypoint() {
+    # Default is to run runit and reconfigure GitLab
+    gitlab-ctl reconfigure &
+    /opt/gitlab/embedded/bin/runsvdir-start &
+    wait
+}
+
+entrypoint