Reduce docker image size

[PrasadG193]

Currently, docker image size is around 572 MB. The major reason behind that is, we are coping source code into the docker base image and building the binary there (https://github.com/infracloudio/botkube/blob/develop/build/Dockerfile). Instead we should be generating binary on local env and copy that binary into the docker image.
So we should have architecture specific build scripts (atleast linux/amd64 for now) which will generate binary and we should modify Dockerfile to just copy generated binary to docker image.
Something like this: https://github.com/fission/fission-workflows/tree/master/build