modify Dockerfile

[jaba0x]

update govendor & add go init cmds

[kgaughan]

This project is essentially dead, but I don't think you'd get it merged anyway. The reason why everything was on one line was to avoid leaving any detritus in the image after the build completed and separating everything out to seperate RUN commands means you're going to end up with a lot of crap left over in the image.

A better route to take would be multistage builds, as that way you're only left with what you actually want in the final image.

On #27, I updated the dockerfile to build the binary separately and copy it into the final container. The final image is ~18MB, and just includes Alpine Linux (7.5MB), the latest ca-certificates package (.5MB), and the bigip_exporter binary itself (10MB).