Harden kubectl checksum verification

Agent-Logs-Url: https://github.com/cloudengine-labs/devops_os/sessions/8929bee2-ec5e-454a-8614-f771484942ab

Co-authored-by: chefgs <7605658+chefgs@users.noreply.github.com>

Author: Copilot

.devcontainer/Dockerfile

@@ -126,7 +126,7 @@ RUN if [ "$INSTALL_KUBECTL" = "true" ]; then \
     KUBECTL_VERSION="$(curl -fsSL https://dl.k8s.io/release/stable.txt)" \
     && curl -fsSLo /tmp/kubectl "https://dl.k8s.io/release/${KUBECTL_VERSION}/bin/linux/amd64/kubectl" \
     && curl -fsSLo /tmp/kubectl.sha256 "https://dl.k8s.io/release/${KUBECTL_VERSION}/bin/linux/amd64/kubectl.sha256" \
-    && echo "$(cat /tmp/kubectl.sha256)  /tmp/kubectl" | sha256sum --check \
+    && test "$(cat /tmp/kubectl.sha256)" = "$(sha256sum /tmp/kubectl | cut -d' ' -f1)" \
     && install -o root -g root -m 0755 /tmp/kubectl /usr/local/bin/kubectl \
     && rm -f /tmp/kubectl /tmp/kubectl.sha256; \
     fi