doc: add instructions for using act

Author: Sjors

ci/README.md

@@ -24,3 +24,29 @@ CI_CONFIG=ci/configs/olddeps.bash  ci/scripts/run.sh
 ```
 
 By default CI jobs will reuse their build directories. `CI_CLEAN=1` can be specified to delete them before running instead.
+
+### Running workflows with `act`
+
+You can run either the entire workflow or a single matrix entry locally. On
+macOS or Linux:
+
+1. Install [`act`](https://github.com/nektos/act) and either Docker or
+  Podman.
+2. Inside the Podman VM, create a named volume for the Nix store (ext4,
+  case-sensitive) so builds persist across runs. Recreate it any time you want
+  a clean cache:
+  ```bash
+  podman volume create libmultiprocess-nix
+  ```
+3. From the repo root, launch the workflow. The example below targets the
+  sanitize matrix entry; drop the `--matrix` flag to run every configuration.
+  ```bash
+  act \
+    --reuse \
+    -P ubuntu-latest=ghcr.io/catthehacker/ubuntu:act-24.04 \
+    --container-options "-v libmultiprocess-nix:/nix" \
+    -j build \
+    --matrix config:sanitize
+  ```
+
+