Add Dockerfile and push public Docker image

[mattbostock]

[davidkarlsen]

I'd be willing to run this in test if an image was pushed to docker-hub.

[mblaschke]

Refactored and implemented this solution on a forked version: https://github.com/webdevops/alertmanager2es

Instead of using busybox this fork is using a distroless container to add more security. Build is done with a multistage Dockerfile.