run GitHub actions locally using gh act

Author: dimpase

src/doc/en/developer/github.rst

@@ -73,6 +73,33 @@ and check::
 
 which will show the default repo along with its readme, which is quite long.
 
+Framework for testing GitHub Actions locally
+--------------------------------------------
+
+A useful extension to ``gh`` allows testing of Sage's GitHub Actions locally,
+using Docker. It is called ``act`` and can be installed as a ``gh extension``::
+
+    [alice@localhost sage]$ gh extension install https://github.com/nektos/gh-act
+
+Extra steps needed for configuration of Docker to run Actions locally can be found on
+`act's GitHub <https://github.com/nektos/act>`_
+
+After it's set up, one can e.g. list all the available actions::
+
+    [alice@localhost sage]$ gh act -l
+    Stage  Job ID                  Job name                                                   Workflow name                                      Workflow file           Events
+    0      build                   build                                                      Build & Test                                       build.yml               workflow_dispatch,pull_request,push
+    0      test                    Conda                                                      Build & Test using Conda                           ci-conda.yml            push,pull_request,workflow_dispatch
+    0      cygwin-stage-i-b        cygwin-stage-i-b                                           CI cygwin-standard                                 ci-cygwin-standard.yml  push,workflow_dispatch
+    [...]
+
+run a particular action ``foobar`` ::
+
+    [alice@localhost sage]$ gh act -j foobar
+    ...
+
+and so on.
+
 
 Linking Git to your GitHub account
 ==================================