Add workflow for building preview release images

[bruth]

This does a multi-arch build for Linux and Windows equivalent to the official releases.

• Extract base image as build-time arg. This is necessary to override the official image name of "nats" to that of "synadia/nats-server" where these preview releases are pushed.

• The update.py script now supports -preview.x or -rc.x pre-release notation when updating the various files under 2.10.x.

Note, 2.9.x can be added as well, but the current scope is only the current minor version.

This has been squashed, but was tested in previous commits, see:

• Linux action
• Windows action

See example pushed images on Docker Hub.

[bruth]

When a tag on nats-docker is pushed, it will create the corresponding Docker images. The end-to-end workflow will be:

• Push a tag in nats-io/nats-server to kick off the binary builds, such as 2.11.0-preview.1
• Create a pre-release release with release notes up to that preview
• Update this repo (nats-docker) such as ./update.py 2.11.0-preview.1, review, and merge
• Push a tag in this repo that then will trigger the Docker image build

Effectively the same workflow as a final release, except for previews we don't open a PR to the official-images repo.

[derekcollison]

LGTM but my docker skills are lackluster at best, so will defer to @wallyqs