Image tags matching the upstream project

[Bekt]

Solves #7

This PR introduces building the image from a different upstream images (FROM). Since there's not a good way to subscribe to specific parent image tag builds, the ci.sh rebuilds images from all available upstream tags.

The end result corresponds to the parent image, meaning this image's tags will be the same as gliderlabs/logspout tags. E.g bekt/logspout-logstash:v3.2.5.

A few ceveats:

1. The produced v1 and v2 images likely do not work as expected, as build.sh was not introduced until v3 in the upstream project.
2. Builds are going to be extremely slow as we are building multiple images instead of a single image now. I'm not sure of Docker Hub automated builds have a timeout. There might be some optimization around with multi-stage builds, but I haven't looked.
3. This project assumes build.sh does not change between versions. A better way would be to download the right build.sh file during build time for each version.

I didn't put much thinking or work into this, just went with the fastest way. If there's a better way, let me know.

[Bekt]

ignore- will be removed after I get some validation this thing works.