Add readme and workflow

.forgejo/workflows/build.yaml

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+on:
+  push:
+  schedule:
+    - cron: '0 12 * * *'
+jobs:
+  build_image:
+    name: Build image
+    env:
+      VERSION: 22.04
+      SCHEDULED: ${{ github.event_name == 'schedule' }}
+      PUSHED: ${{ github.event_name == 'push' }} 
+      REGISTRY: git.korhonen.cc
+      REPO: ${{ env.REGISTRY }}/functionalhacker/ubuntu-act
+    runs-on: docker
+    steps:
+      - name: Check if update available
+        if: env.SCHEDULED
+        id: check
+        uses: https://github.com/ClementTsang/docker-check-base-image-diff@v0.0.2
+        with:
+          base-image: docker://ghcr.io/catthehacker/ubuntu:act-${{ env.VERSION }}
+          derived-image: docker://${{ env.REPO }}:latest
+      - name: Check result
+        if: env.SCHEDULED
+        run: echo "Needs updating = ${{ steps.check.outputs.differs }}"
+      - name: Set up Docker Buildx
+        if: env.PUSHED || ${{ steps.check.outputs.differs == true }}
+        uses: docker/setup-buildx-action@v3
+      - name: Login to Docker registry
+        if: env.PUSHED || ${{ steps.check.outputs.differs == true }}
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+      - name: Build and push image
+        if: env.PUSHED || ${{ steps.check.outputs.differs == true }}
+        uses: docker/build-push-action@v5
+        with:
+          push: true
+          build-args: VERSION=${{ env.VERSION }}
+          tags: |
+            ${{ env.REPO }}:latest
+            ${{ env.REPO }}:${{ env.VERSION }}

README.adoc

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+= Ubuntu Actions runner image
+
+Based on the link:https://github.com/catthehacker/docker_images[ghcr.io/catthehacker/ubuntu:act-22.04] runner image with the following tools added
+
+* link:https://github.com/containers/skopeo[skopeo]
+
+Image is built at 12:00 EET every day if upstream updates are available.