ci: publish user-engine images from forge commits

Add the fleet container-build workflow so latest main can be rolled
out by digest instead of a workstation-built image.

.forgejo/workflows/image.yaml

@@ -0,0 +1,56 @@
+name: Build and Publish Container Image
+
+# Modelled on tenant-engine/.forgejo/workflows/image.yaml. Images are built
+# by CI from a tarball of the pushed commit, never from a workstation tree.
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - ".forgejo/workflows/image.yaml"
+      - "Containerfile"
+      - "src/**"
+      - "pyproject.toml"
+      - "README.md"
+      - "LICENSE"
+  workflow_dispatch:
+
+env:
+  REGISTRY: forgejo.coulomb.social
+  IMAGE_NAME: coulomb/user-engine
+  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}"
+          SHORT="${REF:0:7}"
+          mkdir -p buildctx "${HOME}/bin"
+          wget -qO /tmp/repo.tar.gz \
+            "https://forgejo.coulomb.social/${GITHUB_REPOSITORY}/archive/${SHORT}.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:${PATH}"
+          echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" -u "${REGISTRY_USER}" --password-stdin
+          IMAGE="${REGISTRY}/${IMAGE_NAME}"
+          docker build -f buildctx/Containerfile -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}"
+
+      - name: Report immutable digest
+        run: |
+          set -eu
+          export PATH="${HOME}/bin:${PATH}"
+          IMAGE="${REGISTRY}/${IMAGE_NAME}"
+          SHORT="${GITHUB_SHA:0:7}"
+          docker inspect --format='{{index .RepoDigests 0}}' "${IMAGE}:main-${SHORT}"