Utilize GitHub Actions cache for image builds

[komish]

Description of the change:

• Add a cache-building phase of the release workflow, which uses Docker's GitHub Actions cache backend to cache images
• Modify the goreleaser configuration to leverage said cache when building images

Motivation for the change:

• The Release process has had issues completing from time to time, hitting long timeouts.

  • In doing a deep dive, I think the cause is either (a) storage on runners, or (b) CPU utilization. I can't really diagnose either reliably without running a self-hosted runner, and there's no guarantee that I can replicate.

  • We've seen build times of anywhere from 35 minutes, to as long as 4 hours (after I extended timeouts).

• The project is committed to Goreleaser as a release toolchain.

• If the image build task is separated from the goreleaser task, it stands to reason that most of our challenges will be alleviated, given that each architecture gets its own runner for the build phase.

Alternatives considered:

• I built a separate workflow that involved building the images in a matrix, and then just extracting the installed application and grafting it into the final container using Goreleaser. This is potentially a bit more reliable, but involves building a ton of Dockerfile fragments. It also makes building the image (say, on a workstation) much more challenging.

Additional Benefit

• If the layers are found in cache (e.g. if the changes being made don't change cached layers), the build times are fast. see: https://github.com/komish/ansible-operator-plugins/actions/runs/11281515689 (fully cached).
• This mostly keeps the Dockerfile intact.

Drawbacks:

• The goreleaser file is now very GitHub Actions centric.

[komish]

This has been rebased to address conflicts.