feat: add an LXC based example of docker/build-push-action usage (#781)

The motivations for this additional example are:

- Using https://github.com/docker/build-push-action/ with the Forgejo runner is a recurring question and there is no example of how to do that
- Reproducing https://code.forgejo.org/forgejo/runner/issues/208 will benefit from this example

---

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- features
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/781): <!--number 781 --><!--line 0 --><!--description ZmVhdDogYWRkIGFuIExYQyBiYXNlZCBleGFtcGxlIG9mIGRvY2tlci9idWlsZC1wdXNoLWFjdGlvbiB1c2FnZSBbc2tpcCBjYXNjYWRlXQ==-->feat: add an LXC based example of docker/build-push-action usage [skip cascade]<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/781
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

.forgejo/workflows/docker-build-push-action-in-lxc.yml

@@ -0,0 +1,78 @@
+#
+# Example that requires a Forgejo runner with an [LXC backend](https://forgejo.org/docs/latest/admin/actions/runner-installation/#setting-up-the-container-environment).
+#
+# - Start a Forgejo instance to be used as a container registry
+# - Build a container image using the [docker/build-push-action](https://code.forgejo.org/docker/build-push-action) action
+# - Push the image to the Forgejo instance
+# - Retrieve the image
+#
+# Runs of this workflow can be seen in [the Forgejo runner](https://code.forgejo.org/forgejo/runner/actions?workflow=docker-build-push-action-in-lxc.yml) logs.
+#
+name: example
+on:
+  push:
+    branches:
+      - 'main'
+  pull_request:
+
+env:
+  FORGEJO_VERSION: 11.0.3 # renovate: datasource=docker depName=code.forgejo.org/forgejo/forgejo
+  FORGEJO_USER: root
+  FORGEJO_PASSWORD: admin1234
+
+jobs:
+  docker-build-push-action-in-lxc:
+    if: forge.repository_owner != 'forgejo-integration' && forge.repository_owner != 'forgejo-experimental' && forge.repository_owner != 'forgejo-release'
+    runs-on: lxc-bookworm
+
+    steps:
+
+      - name: install Forgejo so it can be used as a container registry
+        id: registry
+        uses: https://data.forgejo.org/actions/setup-forgejo@v3.0.1
+        with:
+          user: ${{ env.FORGEJO_USER }}
+          password: ${{ env.FORGEJO_PASSWORD }}
+          binary: https://code.forgejo.org/forgejo/forgejo/releases/download/v${{ env.FORGEJO_VERSION }}/forgejo-${{ env.FORGEJO_VERSION }}-linux-amd64
+          lxc-ip-prefix: 10.0.9
+
+      - name: enable insecure / http uploads to the Forgejo registry
+        run: |-
+          set -x
+          # the docker daemon was implicitly installed when Forgejo was
+          # installed in the previous step. But it will refuse to connect
+          # to an insecure / http registry by default and must be told
+          # otherwise
+          cat > /etc/docker/daemon.json <<EOF
+            {
+              "insecure-registries" : ["${{ steps.registry.outputs.host-port }}"]
+            }
+          EOF
+          systemctl restart docker
+
+      - uses: https://data.forgejo.org/docker/setup-qemu-action@v3
+      - uses: https://data.forgejo.org/docker/setup-buildx-action@v2
+        with:
+          # insecure / http connections to the registry are allowed
+          config-inline: |
+            [registry."${{ steps.registry.outputs.host-port }}"]
+              http = true
+
+      - name: login to the Forgejo registry
+        uses: https://data.forgejo.org/docker/login-action@v3
+        with:
+          registry: ${{ steps.registry.outputs.host-port }}
+          username: ${{ env.FORGEJO_USER }}
+          password: ${{ env.FORGEJO_PASSWORD }}
+
+      - name: build and push to the Forgejo registry
+        uses: https://data.forgejo.org/docker/build-push-action@v5
+        with:
+          context: examples/docker-build-push-action
+          push: true
+          tags: ${{ steps.registry.outputs.host-port }}/root/testimage:latest
+
+      - name: verify the image can be read from the Forgejo registry
+        run: |
+          set -x
+          docker pull ${{ steps.registry.outputs.host-port }}/root/testimage:latest

examples/README.md

@@ -1,9 +1,10 @@
-This directory contains a collection of usage and deployment examples.
+A collection of Forgejo actions examples.
 
-Workflow examples can be found [in the documentation](https://forgejo.org/docs/next/user/actions/).
+Other examples can be found [in the documentation](https://forgejo.org/docs/next/user/actions/).
 
 | Section                     | Description                                                                                                                                                                                                                                                   |
 |-----------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| [`docker-build-push-action-in-lxc`](../.forgejo/workflows/docker-build-push-action-in-lxc.yml)              | using the LXC runner backend to build and push a container image using the [docker/build-push-action](https://code.forgejo.org/docker/build-push-action) action                     |
 | [`docker`](docker)              | using the host docker server by mounting the socket                     |
 | [`LXC systemd`](lxc-systemd)              | systemd unit managing LXC containers dedicated to a single runner                     |
 | [`docker-compose`](docker-compose) | all in one docker-compose with the Forgejo server, the runner and docker in docker  |

examples/docker-build-push-action/Dockerfile

@@ -0,0 +1 @@
+FROM code.forgejo.org/oci/alpine:latest

examples/docker-build-push-action/README.md

@@ -0,0 +1,3 @@
+Used by:
+
+- .forgejo/workflows/docker-build-push-action-in-lxc.yml