fix: add kubectl to hub container image (#133)

* fix: trigger image build on git tag push, not only on GitHub Release

The images.yaml workflow only triggered on 'release: types: [created]'
which requires a manually created GitHub Release object. Tags created
via the git refs API don't create a Release, so image builds never fired.

Add 'push: tags: v*' trigger so images build on every version tag.

Co-authored-by: Mangirdas Judeikis <mangirdas@judeikis.lt>

* fix: add kubectl to hub container image

kubectl is needed for hub operations against tenant clusters.
Downloads the latest stable kubectl during image build, matching
the target platform architecture (amd64/arm64).

Co-authored-by: Mangirdas Judeikis <mangirdas@judeikis.lt>

---------

Co-authored-by: mjudeikis-bot <ai@faros.sh>
Co-authored-by: Mangirdas Judeikis <mangirdas@judeikis.lt>

Author: 3 people

deploy/Dockerfile.hub

@@ -21,7 +21,11 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build \
     -o /kedge-hub ./cmd/kedge-hub/
 
 FROM alpine:3.21
-RUN apk add --no-cache ca-certificates
+ARG TARGETARCH
+RUN apk add --no-cache ca-certificates curl && \
+    KUBECTL_VERSION=$(curl -fsSL https://dl.k8s.io/release/stable.txt) && \
+    curl -fsSL "https://dl.k8s.io/release/${KUBECTL_VERSION}/bin/linux/${TARGETARCH}/kubectl" -o /usr/local/bin/kubectl && \
+    chmod +x /usr/local/bin/kubectl
 COPY --from=builder /kedge-hub /kedge-hub
 USER 65534:65534
 ENTRYPOINT ["/kedge-hub"]