Docker Build action is missing some modernities

[kingdonb]

I'm dogfooding some of my docs, and this one has gone a bit stale:

https://fluxcd.io/flux/use-cases/gh-actions-app-builder/

It needs at least:

! The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
docker: .github#248

and

permissions:
  packages: write

from a first pass – I'll come back to this later, but right now I just wanted to be sure I don't lose track from this because I'm moving on just this minute...

[kingdonb]

We can add a trigger to this workflow task so that it runs whenever netlify.toml is updated:

https://github.com/fluxcd/website/blob/main/.github/workflows/build-push-hugo-image.yml
which must run first, followed by
https://github.com/fluxcd/website/blob/main/.github/workflows/build-push-hugo-support.yml

I think my GitHub Actions-fu is strong enough at this point to set them up as an ordered set of jobs that runs on the list of files changes, in the correct dependency order.

I'm going to check out this week if this really needs to be run every time or if it will still work with an older version, make up a list of files (like netlify.toml, Dockerfile, etc.) that should be included in the list of files that trigger a new hugo image and new support image to be built. Also, we can look into automating the hugo upgrade PR with dependabot or something else. I'm partial to Flux image update automation, but I don't think we can use it to write into a .toml file!