run GitHub Actions workflows locally in Docker using act

.github/act/README.md

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+# Running spamassassin_make_test.yml locally with `act`
+
+[`act`](https://github.com/nektos/act) runs GitHub Actions workflows locally in Docker, so you can test
+`.github/workflows/spamassassin_make_test.yml` without pushing to GitHub.
+
+## Files
+
+Copy each `.dist` template to drop the `.dist` suffix, then edit as needed:
+
+```bash
+cp .github/act/act-event.json.dist .github/act/act-event.json
+cp .github/act/act.env.dist .github/act/act.env
+```
+
+- `act-event.json` — the `workflow_dispatch` event payload. Set `inputs.runners`, `inputs.perls`,
+  `inputs.database` (each a JSON-encoded string, matching the workflow's own input defaults) to pick
+  which OS/perl/database combinations to run, and `inputs.tests` to a `t/*.t` glob to restrict which
+  tests run.
+- `act.env` — environment variables passed into the job containers, e.g. `PERL_MM_USE_DEFAULT=1`.
+
+## Run
+
+```bash
+# dry run — show what would execute without running it
+act -n -e .github/act/act-event.json --env-file .github/act/act.env
+
+# real run
+act -e .github/act/act-event.json --env-file .github/act/act.env
+```

.github/act/act-event.json.dist

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+{
+  "inputs": {
+    "runners": "[\"ubuntu-latest\"]",
+    "perls": "[40]",
+    "database": "[\"none\"]",
+    "tests": ""
+  }
+}

.github/act/act.env.dist

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+PERL_MM_USE_DEFAULT=1