Create multi-stage Dockerfile for easy builds

[todaywasawesome]

Just what the title says. The multi-stage part means we use separate containers for the build and the "runtime" so to speak. This way we can keep the image nice and small.

This also makes building the app very easy because you just need the Dockerfile, whereas before it relies on the user building the binary in the same directory as the binary.

You can see the image built here: https://hub.docker.com/r/todaywasawesome/kube-plex

[munnerz]

This looks great, and certainly makes builds easier 😄 could you remove the codefresh specific changes here for now? We don't currently have a CI system set up, but I think we're more likely to use something like GitHub actions as it saves having to configure additional external services 😄

[todaywasawesome]

@munnerz PR updated to remove the files you didn't want. Should be good to go now.