ci: build with buildctl instead of buildah (infra#241)

Rootless buildah in gitea-runner's hardened securityContext (capabilities:
drop [ALL], allowPrivilegeEscalation: false) cannot remap the UID/GID range
newuidmap/newgidmap need on a fresh base-image pull, and the pod has no OCI
runtime at all. buildctl talks to the host's buildkitd over a socket, so the
privileged layer extraction happens on the host and the pod's securityContext
stops being in the picture. Same pattern as mathias/infra's segment-embedder.yml
and postgres18.yml.

A template is the highest-leverage place to fix this. Every repo generated from
here inherits its CI, so a buildah-based template does not just carry the bug —
it manufactures new instances of it. A fleet inventory for infra#241 found nine
repos still invoking buildah, and two of them are the templates.

Note the failure is latent, not immediate: warm base-image layers mask it, so a
generated repo builds fine until the first genuinely cold pull. That is exactly
the situation a NEW repo is in.

Verified by running the exact command against this repo's real build context:

    buildctl --addr unix:///run/buildkit/buildkitd.sock build \
      --frontend dockerfile.v0 --local context=. --local dockerfile=. \
      --output type=oci,dest=$TAR
    -> exporting manifest sha256:5a841529... DONE, 6.8MB OCI archive

That check matters here because this repo's own CI cannot validate the change:
the build job is deliberately skipped on the raw template (2ba2698), since
__PROJECT_NAME__ is not a legal image name. The next generated repo is the first
place it runs for real.

skopeo copy replaces `buildah push` — buildctl has no registry-push equivalent —
and runs twice to preserve the :sha and :latest tags the previous version pushed.

.gitea/workflows/cd.yml

@@ -54,18 +54,41 @@ jobs:
           SHA=$(git rev-parse --short HEAD)
           echo "sha-tag=${SHA}" >> "$GITHUB_OUTPUT"
 
+      # infra#241: buildctl, NOT buildah. Rootless buildah in gitea-runner's
+      # hardened securityContext (capabilities: drop [ALL],
+      # allowPrivilegeEscalation: false) cannot remap the UID/GID range
+      # newuidmap/newgidmap need on a fresh base-image pull, and it has no OCI
+      # runtime at all. buildctl talks to the host's buildkitd over a socket, so
+      # the privileged extraction happens in buildkitd on the host and never in
+      # this pod. Same pattern as mathias/infra's segment-embedder.yml.
+      #
+      # This matters MORE in a template than anywhere else: every repo generated
+      # from here inherits its CI, so a buildah-based template manufactures new
+      # instances of a bug that has already cost this estate several red builds.
       - name: Build and push to local registry
         run: |
+          set -e
+          BUILDKIT_HOST="unix:///run/buildkit/buildkitd.sock"
           REGISTRY="localhost:5000"
           REF="${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.sha-tag }}"
-          buildah build \
+          TAR_FILE="$(mktemp)"
-            --label "org.opencontainers.image.revision=${{ github.sha }}" \
+          trap 'rm -f "$TAR_FILE"' EXIT
-            -t ${REF} \
+
-            -t ${REGISTRY}/${{ env.IMAGE }}:latest \
+          buildctl --addr "$BUILDKIT_HOST" build \
-            .
+            --frontend dockerfile.v0 \
-          buildah push --tls-verify=false ${REF}
+            --local context=. \
-          buildah push --tls-verify=false ${REGISTRY}/${{ env.IMAGE }}:latest
+            --local dockerfile=. \
-          echo "✓ Image pushed to ${REF}"
+            --opt label:org.opencontainers.image.revision="${{ github.sha }}" \
+            --output type=oci,dest="$TAR_FILE"
+
+          # buildctl has no registry-push equivalent to `buildah push`; skopeo
+          # copies the OCI archive up. Two copies because the buildah version
+          # tagged both :sha and :latest.
+          skopeo copy --dest-tls-verify=false \
+            oci-archive:"$TAR_FILE" docker://${REF}
+          skopeo copy --dest-tls-verify=false \
+            oci-archive:"$TAR_FILE" docker://${REGISTRY}/${{ env.IMAGE }}:latest
+          echo "✓ Image pushed to ${REF} and :latest"
 
   deploy:
     name: Deploy via GitOps