Improve the build time

[wliao008]

Just a placeholder, I attempted it once with no result and gave up shortly after, but would like to get that looked at some point.

Building registrator locally takes 5+ minutes every time for me, this is pretty frustrating when testing just even a small change, as it goes thru the whole process of building the docker image, downloading and setting up the entire toolchain so it can build the go app.

One options I was thinking about: skip the toolchain setup in Dockerfile, instead build the go app on the local machine, and add the executable to the docker image (sumosidecar is taking a similar approach).

However there is one obstacle:

registrator contains hard references to gliderlabs/registrator, when building outside of the container, it does not find code written by external parties (such as eureka/eureka.go). At build time, we could possibly copy the entire dir somewhere else, set the GOPATH, do the build, and revert back the GOPATH, but that sounds kinda convoluted.

[jamiegs]

When testing if a build completes successfully, I just use go build