NO-JIRA: refactor(.github/workflows): use JSON-based platform runner mapping and add README for runners, then add IBM runners

Author: jiridanek

.github/README.md

@@ -0,0 +1,19 @@
+## Available runners
+
+List for
+[public repositories](https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for-public-repositories)
+and for
+[private repositories](https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for--private-repositories).
+
+- **Linux**: `ubuntu-latest`, `ubuntu-24.04`, `ubuntu-24.04-arm`
+
+We have [IBM PowerPC and Z runners](https://community.ibm.com/community/user/blogs/elizabeth-k-joseph1/2025/05/19/expanding-open-source-access-hosted-github-actions) available through
+
+* https://github.com/IBM/actionspz/issues/63
+
+- **Linux**: `ubuntu-24.04-ppc64le`, `ubuntu-24.04-s390x`
+
+We have considered investigating custom runners, either just plain containers/VMs, or something fronting an OpenShift cluster, in
+
+* https://github.com/opendatahub-io/notebooks/issues/1389
+* https://github.com/opendatahub-io/notebooks/pull/2627

.github/workflows/build-notebooks-TEMPLATE.yaml

@@ -32,7 +32,10 @@ name: Build & Publish Notebook Servers (TEMPLATE)
 jobs:
   build:
     # https://docs.github.com/en/actions/how-tos/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
-    runs-on: ${{ inputs.platform == 'linux/arm64' && 'ubuntu-24.04-arm' || 'ubuntu-24.04' }}
+    runs-on: >-
+      ${{ fromJSON(
+        '{"linux/amd64": "ubuntu-24.04", "linux/arm64": "ubuntu-24.04-arm", "linux/ppc64le": "ubuntu-24.04-ppc64le", "linux/s390x": "ubuntu-24.04-s390x"}'
+      )[inputs.platform] || 'no-such-runner' }}
     env:
       # Some pieces of code (image pulls, for example) in podman consult TMPDIR or default to /var/tmp
       TMPDIR: /home/runner/.local/share/containers/tmpdir