Use multi-stage builder in single Dockerfile to simplify image build

[YanzheL]

What does this PR do

Multi-stage builder in single Dockerfile

This simplifies the process of building docker images. So we don't need to write complex procedures to do it in Makefile with separate intermediate containers. Users can use this single Dockerfile to create aledbf/kube-keepalived-vip image

Update golang version to 1.14

Golang 1.14 has various performance improvements in runtime and compiler, especially in timer, locks and goroutines (Huge improvement).
See Go 1.14 Release Notes

Update keepalived to v2.0.20

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 15.014% when pulling 4ff0321 on YanzheL:patch-docker into 121232b on aledbf:master.

[YanzheL]

I already tested and deployed this new image in my cluster. LGTM.

[YanzheL]

@aledbf