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.

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 merely carry the bug,
it manufactures new instances. The failure is latent — warm base-image layers
mask it — so a generated repo builds fine until its first genuinely cold pull,
which is precisely the situation a new repo is in.

Verification is PARTIAL here, and the limit is worth stating rather than
implying a green build. Running the exact command against this repo's real
context reaches step 8 and fails inside the Dockerfile:

    RUN templ generate && go build ... -o /out/app ./cmd/__PROJECT_NAME__
    error: failed to solve: ... exit code: 1

That failure is the template's own placeholder path, which cannot resolve on the
raw template — the same reason the guard job gates check/build/deploy off here
(is_template detects __MODULE_PATH__ in go.mod). What it does prove is that
buildctl resolved the frontend, the context and the Dockerfile and began
executing stages; every part of the buildctl integration worked, and only the
template's unsubstituted content did not.

template-go-agent's Dockerfile has no placeholder in its build path, so there the
same command produced a complete 6.8MB OCI archive. The first generated repo from
this template is where this one gets its end-to-end proof.

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

.gitea/workflows/cd.yml

@@ -67,16 +67,38 @@ jobs:
 
       - name: Build and push to local registry
         run: |
+          set -e
+          # infra#241: buildctl, NOT buildah. Rootless buildah in gitea-runner's
+          # hardened securityContext 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 extraction happens on the host. Same
+          # pattern as mathias/infra's segment-embedder.yml.
+          #
+          # Highest-leverage place to fix it: every repo generated from this
+          # template inherits this CI, so a buildah-based template manufactures
+          # new instances of the bug. Warm layers mask it, so a generated repo
+          # builds fine until its first genuinely cold base-image pull.
+          BUILDKIT_HOST="unix:///run/buildkit/buildkitd.sock"
           REGISTRY="localhost:5000"
           REF="${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.sha-tag }}"
-          buildah build \
-            --label "org.opencontainers.image.revision=${{ github.sha }}" \
-            -t ${REF} \
-            -t ${REGISTRY}/${{ env.IMAGE }}:latest \
-            .
-          buildah push --tls-verify=false ${REF}
-          buildah push --tls-verify=false ${REGISTRY}/${{ env.IMAGE }}:latest
-          echo "✓ Image pushed to ${REF}"
+          TAR_FILE="$(mktemp)"
+          trap 'rm -f "$TAR_FILE"' EXIT
+
+          buildctl --addr "$BUILDKIT_HOST" build \
+            --frontend dockerfile.v0 \
+            --local context=. \
+            --local dockerfile=. \
+            --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. Twice, to keep the :sha and :latest tags.
+          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