Create and publish nightly docker images

**Is your enhancement request related to a problem? Please describe.**
Currently Docker Hub only contains release images for Pulsar. This makes doing CI for dependent repos harder, since they wouldn't be able to test again the bleeding edge easily, and has lead to issues such as https://github.com/apache/pulsar-client-go/issues/748 where it's unclear which Pulsar change caused the issue.

(The linked issue was made worse because the `latest` image has been outdated by ~6 months, but 1) even if that's not the case we'd still only be able to catch issues between releases, and 2) the fact that the Golang client is using `latest` points to a need of having a nightly.)

**Describe the solution you'd like**
Create a nightly build so external repos can always point against it and catch issues as early as possible.

**Describe alternatives you've considered**
The external repos can also try to pull in the latest Pulsar code, but that seems harder and needs more per repo configuration.

Review Pulsar's existing release Docker image publishing setup and determine where a nightly build would fit. Check how images are currently published to Docker Hub and define the nightly image tag and update behavior needed by external repositories. Done means external repositories can reliably pull a current nightly image.

Written by the indexing model from the issue text.