Use tini as a minimal init for the docker image

[noushi]

This PR fixes #12 (so far).

It's based on alpine:3.4 to minimize image size.

Please notice that I only modified the docker make target, docker-build will need to be modified as well.

[wrouesnel]

So I don't know what to do about this one because it's an issue I can't replicate anywhere else.

[noushi]

Aside from the bug we experienced and solved using this PR, it's better to have both:

• a Linux environment one can use to enter a running container
• signal handling using an init-compatible entrypoint

Basically you have two ways of managing signals:

• you use an init-compatible entrypoint
• you handle the signals yourself in your entrypoint (postgres_exporter in our case)

That said, although Prometheus (server) handles signals itself (as shown here and here), it still uses a minimal base image, with regular Linux tools, like sh, ps, cat...

If not for the bug fix, this PR serves as two feature enhancements:

• signal handling
• OS environment in image

[wrouesnel]

So I'm going to close this for now, as the underlying issue I haven't been able to replicate. Closes #12 as well.

My preferred fix for this is to handle the correct signals in the postgres_exporter binary, which as far as I know does happen at the moment (in the sense that I have not been able to replicate the issue locally).

[noushi]

Sure, thank you @wrouesnel .