feat(ci): prefer hosted runners, fallback to self-hosted via reusable workflows

[polyipseity]

Summary

• Add reusable workflows for release, docker, and checks; callers now use hosted-first → self-hosted fallback.
• New files: .github/workflows/{release,docker,check}-reusable.yml.
• Callers updated: .github/workflows/{release,docker,check}.yml call reusable workflows for try/fallback.
• Fallback gating uses if: ${{ failure() }} so fallback runs only when the try job fails.
• continue_on_error is passed into reusable workflows as an input and applied at job level.
• Clarifying comments added about PR UI duplication and cosmetic failures when fallback runs.
• Fallback runs_on set to self-hosted.

Notes

• Branch: feat/prefer-hosted-fallback-reusable
• Expect PR UI to show duplicated/expanded checks when fallback executes; this is cosmetic (documented in comments).

[flandia]

LGTM, ready for merge?

[polyipseity]

Yes, except that maybe we could use a better name than *-reusable.yml (seems to wrongly imply the workflows are reusable in some way, whereas it reality it just avoids duplicating the same steps twice).

[polyipseity]

Renamed the workflow to a better name (shared workflow), and the MongoDB step is from the commented out step (https://github.com/USThing/template-api/pull/48/changes#diff-55f05888fd854d9962bdd4d8ccf23a07b610990f58174397b2586b58f8937428L59-L61).