MEIER-326: Add Docker GH Actions cache to CI workflows

[andymeierdev]

Summary

Adds Docker build layer caching via GitHub Actions cache to both CI workflows (deploy.yml and preview.yml), matching the pattern already used in the agent repo's Pulumi workflows.

Changes

.github/workflows/deploy.yml

• Added Setup Docker Buildx step (docker/setup-buildx-action@v3) to enable the buildx builder
• Added Expose GitHub Actions cache env step to export ACTIONS_CACHE_URL and ACTIONS_RUNTIME_TOKEN environment variables, allowing Docker builds triggered by Pulumi to leverage GHA cache for layer caching

.github/workflows/preview.yml

• Same two steps added to the preview job, before the Pulumi authenticate step

Validation

• Verified both workflow files match the caching pattern from ../agent/.github/workflows/deploy.yml and preview.yml
• YAML structure reviewed for correct indentation and step ordering

[github-actions]

🍹 preview on andymeier/prod

Pulumi report

View in Pulumi Cloud

  Previewing update (prod)

View Live: https://app.pulumi.com/meiermade/andymeier/prod/previews/613ece2b-f02f-4104-981b-a1513af70306

pulumi:pulumi:Stack: (same)
  [urn=urn:pulumi:prod::andymeier::pulumi:pulumi:Stack::andymeier-prod]
Resources:
  16 unchanged