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

Rootless buildah in the containerized gitea-runner pod cannot remap
UID/GID on a fresh base-image pull and has no OCI runtime, so this
Dockerfile's `RUN apk add` / `RUN go build` cannot execute there at all.
The last green run here (2026-07-22) predates the runner's
containerization (infra#233, 2026-07-29), so the build has been latently
broken since.

buildctl hands the build to koala's host buildkitd over the socket the
pod already mounts; skopeo pushes the resulting OCI archive once per tag,
since buildctl has no `buildah push` equivalent. The VERSION build-arg
carries over as `--opt build-arg:VERSION=`, so the ldflags version stamp
is unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011PL5XcG68SvQqz3vGVbWbf

.gitea/workflows/cd.yml

@@ -62,24 +62,48 @@ jobs:
 
       - name: Build and push to local registry
         run: |
+          set -e
+          # 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 the pod has
+          # no OCI runtime at all -- so this Dockerfile's `RUN apk add` / `RUN go
+          # build` could not execute there either. buildctl is a thin client
+          # that hands the build to the host's buildkitd over a socket, so the
+          # privileged layer work happens on the host and this pod's
+          # securityContext stops being in the picture. Same pattern as
+          # mathias/infra's segment-embedder.yml.
+          BUILDKIT_HOST="unix:///run/buildkit/buildkitd.sock"
           REGISTRY="localhost:5000"
           REF="${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.sha-tag }}"
           # Stamp the real build version: the git tag on a v* build, else the short sha.
           VERSION="${{ steps.meta.outputs.version-tag }}"
-          [ -z "$VERSION" ] && VERSION="${{ steps.meta.outputs.sha-tag }}"
-          buildah build \
-            --build-arg VERSION="${VERSION}" \
-            --label "org.opencontainers.image.revision=${{ github.sha }}" \
-            --label "org.opencontainers.image.source=${{ github.repositoryUrl }}" \
-            -t ${REF} \
-            -t ${REGISTRY}/${{ env.IMAGE }}:latest \
-            .
-          buildah push --tls-verify=false ${REF}
-          buildah push --tls-verify=false ${REGISTRY}/${{ env.IMAGE }}:latest
-          [[ -n "${{ steps.meta.outputs.version-tag }}" ]] && \
-            buildah push --tls-verify=false \
-              ${REF} \
-              ${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.version-tag }} || true
+          # `if`, not `[ -z ] &&`: under `set -e` the && form exits the step
+          # whenever VERSION is already set.
+          if [ -z "$VERSION" ]; then VERSION="${{ steps.meta.outputs.sha-tag }}"; fi
+          TAR_FILE="$(mktemp)"
+          trap 'rm -f "$TAR_FILE"' EXIT
+
+          buildctl --addr "$BUILDKIT_HOST" build \
+            --frontend dockerfile.v0 \
+            --local context=. \
+            --local dockerfile=. \
+            --opt build-arg:VERSION="${VERSION}" \
+            --opt label:org.opencontainers.image.revision="${{ github.sha }}" \
+            --opt label:org.opencontainers.image.source="${{ github.repositoryUrl }}" \
+            --output type=oci,dest="$TAR_FILE"
+
+          # buildctl has no registry-push equivalent to `buildah push`; skopeo
+          # copies the OCI archive up, once per tag.
+          skopeo copy --dest-tls-verify=false \
+            oci-archive:"$TAR_FILE" docker://${REF}
+          skopeo copy --dest-tls-verify=false \
+            oci-archive:"$TAR_FILE" docker://${REGISTRY}/${{ env.IMAGE }}:latest
+          if [ -n "${{ steps.meta.outputs.version-tag }}" ]; then
+            skopeo copy --dest-tls-verify=false \
+              oci-archive:"$TAR_FILE" \
+              docker://${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.version-tag }}
+          fi
           echo "✓ Image pushed to ${REF}"
 
       - name: Smoke test