Add versioning, tagging, docker push

[lrettig]

We ran into an issue today where after #105 was merged, go-spacemesh system tests started failing because they don't pin the poet version (see spacemeshos/go-spacemesh#2189). In order to facilitate pinning, we should add versioning here and set up automatic dockerbuild/push when a new version is merged or a new release is cut.

[poszu]

Closing as already implemented.