ci: add publish workflow for backend

[andyatmiami]

related: #607

This commit adds a GHA workflow to support publishing a container image for the workspaces/backend component as well as some updates to the Makefile to establish reasonable (but overridable) defaults.

Key behavior:

• Publishes backend image on any workspaces/ directory change
• Builds image with commit SHA tags by default
  • "release" images with use the tag as defined by the VERSION file
  • a -dirty suffix is added to the tag if the codebase is not porcelain
  • no latest tag is ever produced on images
• Uses ghcr.io/kubeflow/notebooks registry with configurable naming

example image name from "random" commit:

• ghcr.io/kubeflow/notebooks/workspaces-backend:sha-3fa851ab3173942dbaa1a609468e7f9eadf5f4e4

example image name from release:

• ghcr.io/kubeflow/notebooks/workspaces-backend:v2.0.0

[google-oss-prow]

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[andyatmiami]

/ok-to-test

[andyatmiami]

Verification on my fork for "typical" code changes:

• https://github.com/andyatmiami/kubeflow-notebooks/actions/runs/17926340823/job/50973213931

[andyatmiami]

Verification on my fork for "release" code changes:

• https://github.com/andyatmiami/kubeflow-notebooks/actions/runs/17926876094/job/50975069043

[andyatmiami]

/hold

likely will close this unmerged as #605 now handles the logic introduced here.. but will wait until early next week while I test #605

[andyatmiami]

/close

A superior (and cross cutting) solution is being worked as part of #605

[google-oss-prow]

@andyatmiami: Closed this PR.

In response to this:

/close

A superior (and cross cutting) solution is being worked as part of #605

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