Added github actions to build and publish docker image

[D3N14L]

This adds a github action to build and publish the docker image to docker hub.

It just requires two secrets to be set on the github repo:

• DOCKERHUB_USERNAME
• DOCKERHUB_TOKEN

The action is only triggered on publish events of a release on the github project.

Having the docker images published to docker hub would be very helpful during the devlopment of my helm chart.

[D3N14L]

This solves #16 .

[D3N14L]

@ldurnez @ncrocfer trying to move further with the helm chart, but the missing docker containers are a bit of a blocker... Any chance you could review this "sometime soon"?