Add repository-dispatch event on merge to flux-core master branch

[SteVwonder]

Problem: flux-sched's docker image is only re-built when a new commit lands in the flux-sched master branch. It would be cool if the flux-sched images were re-built and deployed with the latest version of flux-core after each merge into flux-core's master branch.

This can be accomplished with Github's webhooks. There is a github action that we can add to flux-core to make this quite simple: https://github.com/peter-evans/repository-dispatch

We would then need to add a new workflow action to flux-sched that rebuilds and deploys the updated images.