add the docker-action example & test

Refs: https://code.forgejo.org/forgejo/runner/issues/49

.forgejo/workflows/integration-nested.yml

@@ -1,6 +1,6 @@
 on: [ push, pull_request ]
 jobs:
-  integration:
+  integration-nested:
     runs-on: self-hosted
     steps:
       - uses: actions/checkout@v3

.forgejo/workflows/integration.yml

@@ -7,7 +7,7 @@ env:
   # Forgejo instance. The test will be a success once the status of
   # the commit is success, as set by the Forgejo Action run.
   #
-  TESTS: 'echo service container expression local-action'
+  TESTS: 'echo service container expression local-action docker-action'
 
 jobs:
   integration:
@@ -18,6 +18,14 @@ jobs:
           set -x
           LXC_IP_PREFIX=10.0.10 ./forgejo-dependencies.sh
           ./forgejo.sh setup root admin1234 codeberg.org/forgejo/forgejo 1.19
+          #
+          # Uncomment the following for a shortcut to debugging the Forgejo runner.
+          # It will build the runner from a designated repository and branch instead of
+          # downloading it from a canonical release.
+          #
+          #./forgejo-test-helper.sh build_runner http://code.forgejo.org/earl-warren/runner wip-sync
+          #export PATH=$(pwd)/forgejo-runner:$PATH
+          #
           ./forgejo-runner.sh setup
           export FORGEJO_RUNNER_LOGS=forgejo-runner.log
           for example in $TESTS ; do

testdata/example-docker-action/.forgejo/workflows/test.yml

@@ -0,0 +1,19 @@
+on: [push]
+
+jobs:
+  ls:
+    runs-on: docker
+    steps:
+      #
+      # This Docker action creates the file SOMEFILE ...
+      #
+      - uses: https://code.forgejo.org/forgejo/test-setup-forgejo-docker@main
+        with:
+          args: ${{ github.workspace }}/SOMEFILE
+      #
+      # ... which then also exists in the job workspace
+      # because both docker containers are automatically
+      # sharing the volume that contains it.
+      #
+      - run: |
+          test -f ${{ github.workspace }}/SOMEFILE