CI: Forgejo Actions examples

For more information see the documentation

https://forgejo.org/docs/v1.21/user/actions/

It is still incomplete and you can take a look at the GitHub Actions
for inspiration. Note however that there is no guarantee they are
compatible and each new thing should be tested first.

.forgejo/workflows/demo-docker.yml

@@ -0,0 +1,8 @@
+on: [pull_request]
+jobs:
+  demo:
+    runs-on: docker
+    steps:
+      - uses: actions/checkout@v4
+      - run: |
+          uname -a

.forgejo/workflows/demo-lxc.yml

@@ -0,0 +1,10 @@
+on: [pull_request]
+jobs:
+  demo:
+    runs-on: k8s
+    steps:
+      - run: |
+          set -x
+          apt-get update -qq
+          apt-get -q install -qq docker.io
+          docker info