Reduce Docker image size by switching to alpine

[JensRantil]

I notice that the Docker image uses golang:1.12. It is based on Debian Stretch and is ~296 MB. How about switching to use 1.12-alpine which is 128 MB? There would be a 168 MB saving which would be a nice improvement to those of use who are using the Docker image for CI (less to download => faster CI step execution).

[volmedo]

I agree with @JensRantil

Aside from that, what about using ENTRYPOINT in the Dockerfile instead of CMD? The usual convention for this kind of docker images is to call the tool passing the usual set of params to docker run without having to write the name of the command again.