Tier-2 Forgejo pilot: image build workflow on railiance01 runner

Uses archive checkout + static docker-cli (non-root host runner pattern).
Pushes to forgejo.coulomb.social/coulomb/key-cape per RAIL-HO-WP-0005-T10.

.forgejo/workflows/image.yaml

@@ -0,0 +1,43 @@
+name: Build and Publish Container Image
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - ".forgejo/workflows/image.yaml"
+      - "Dockerfile"
+      - "src/**"
+  workflow_dispatch:
+
+env:
+  REGISTRY: forgejo.coulomb.social
+  IMAGE_NAME: coulomb/key-cape
+  DOCKER_HOST: tcp://127.0.0.1:2375
+
+jobs:
+  build-and-push:
+    runs-on: container-build
+    steps:
+      - name: Build and push image
+        env:
+          REGISTRY_USER: ${{ secrets.REGISTRY_USER }}
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+        run: |
+          set -eu
+          REF="${GITHUB_SHA:-main}"
+          mkdir -p buildctx "${HOME}/bin"
+          wget -qO /tmp/repo.tar.gz \
+            "https://forgejo.coulomb.social/${GITHUB_REPOSITORY}/archive/${REF}.tar.gz"
+          tar xzf /tmp/repo.tar.gz -C buildctx --strip-components=1
+          wget -qO- https://download.docker.com/linux/static/stable/x86_64/docker-27.3.1.tgz \
+            | tar xz --strip-components=1 -C "${HOME}/bin" docker/docker
+          export PATH="${HOME}/bin:${PATH}"
+          docker version
+          echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" -u "${REGISTRY_USER}" --password-stdin
+          SHORT="${REF:0:7}"
+          IMAGE="${REGISTRY}/${IMAGE_NAME}"
+          docker build -t "${IMAGE}:latest" -t "${IMAGE}:main-${SHORT}" buildctx
+          docker push "${IMAGE}:latest"
+          docker push "${IMAGE}:main-${SHORT}"
+          echo "pushed ${IMAGE}:latest and ${IMAGE}:main-${SHORT}"