reusable workflow for bake

[crazy-max]

Similar to #4, this adds a reusable workflow for bake.

Last commit adds an artifact-name input to avoid conflicts if reusable workflow is used in multiple jobs otherwise it would fail with: https://github.com/docker/github-builder-experimental/actions/runs/19328579362/job/55285747986#step:15:18

With the provided path, there will be 6 files uploaded
Artifact name is valid!
Root directory input is valid!
Error: Failed to CreateArtifact: Received non-retryable error: Failed request: (409) Conflict: an artifact with this name already exists on the workflow run

Also adds a cache-scope input defaulting to build target name.

[crazy-max]

Getting this one in to start working on #23 and continue #22