diff --git a/skills/gitea-ci/SKILL.md b/skills/gitea-ci/SKILL.md
index 837ac16..ae5f0ac 100644
--- a/skills/gitea-ci/SKILL.md
+++ b/skills/gitea-ci/SKILL.md
@@ -1,6 +1,6 @@
 ---
 name: gitea-ci
-description: End-to-end Gitea CI/CD setup with self-hosted act_runner, quality gate pipeline, buildctl image build, k3s deploy, and GitHub mirror. Covers act_runner-specific gotchas that are not documented anywhere.
+description: End-to-end Gitea CI/CD setup with self-hosted act_runner, quality gate pipeline, shared buildctl image-build script, k3s deploy, and GitHub mirror. Covers act_runner-specific gotchas that are not documented anywhere.
 ---
 
 # Gitea CI/CD Pipeline Setup
@@ -133,44 +133,39 @@ jobs:
             echo "version-tag=${REF#refs/tags/}" >> "$GITHUB_OUTPUT"
           fi
 
+      # infra#309: do NOT paste a build implementation here. The estate has
+      # exactly one, mathias/infra's ci-templates/build-image.sh, and each
+      # repo carries a byte-identical copy at .gitea/build-image.sh that a
+      # drift gate keeps current.
+      #
+      # Copying the implementation is what made the buildah->buildctl
+      # migration (infra#241) an eleven-repo sprint, and what left this very
+      # skill handing out the broken version for weeks after the app repos
+      # were fixed. Per-repo differences are INPUTS, not edits to the copy.
+      #
+      # To adopt in a new repo:
+      #   1. add the repo to mathias/infra ci-templates/adopters.txt
+      #   2. run `task ci:sync` in mathias/infra -- it writes the copy
+      #   3. commit .gitea/build-image.sh in your repo
+      #   4. call it as below
+      #
+      # Skipping step 1 leaves the copy unmanaged: it works today and falls
+      # silently behind the next time the canonical file changes.
+      #
+      # Inputs: IMAGE (required, usually the workflow-level env), SHA_TAG,
+      # VERSION_TAG, GIT_SHA, IMAGE_SOURCE, BUILD_ARGS, EXTRA_LABELS,
+      # CONTEXT, DOCKERFILE_DIR, DOCKERFILE_NAME, PUSH_LATEST. Read the
+      # canonical file's header for the full list.
+      #
+      # `bash <script>`, not a direct exec: a file written through the Gitea
+      # API lands 0644, and CI that depends on a file mode is a trap.
       - name: Build and push to local registry
-        run: |
-          set -e
-          # buildctl, NOT buildah (infra#241). The runner is a pod with
-          # `capabilities: drop [ALL]` and `allowPrivilegeEscalation: false`,
-          # so rootless buildah cannot remap UID/GID for a fresh base-image
-          # pull and the pod has no OCI runtime -- every `RUN` step fails.
-          # buildctl is a thin client: it hands the build to the host's
-          # buildkitd over a socket, so the privileged work happens on the
-          # host and the pod's securityContext is not in the picture.
-          # The failure is LATENT -- warm base-image layers mask it, so a
-          # buildah step can look fine for weeks and then fail on a cold pull.
-          BUILDKIT_HOST="unix:///run/buildkit/buildkitd.sock"
-          REGISTRY="localhost:5000"
-          REF="${REGISTRY}/${{ env.IMAGE }}:${{ steps.meta.outputs.sha-tag }}"
-          TAR_FILE="$(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="${{ github.sha }}" \
-            --opt label:org.opencontainers.image.source="${{ 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}"
+        env:
+          SHA_TAG: ${{ steps.meta.outputs.sha-tag }}
+          VERSION_TAG: ${{ steps.meta.outputs.version-tag }}
+          GIT_SHA: ${{ github.sha }}
+          IMAGE_SOURCE: ${{ github.repositoryUrl }}
+        run: bash .gitea/build-image.sh
 
       # No smoke test step. The one this skill used to recommend ran
       # `sudo k3s ctr run` against the host's containerd -- that worked when