ci: add temporary Net debug step for MTU/connectivity diagnosis

.crow/build.yaml

@@ -87,6 +87,31 @@ steps:
   #     - git merge upstream/main --allow-unrelated-histories
   #.
 
+  - name: Net debug
+    image: reg.devxy.io/docker.io/library/alpine:3.23
+    commands:
+      - echo "=== pod interface ==="
+      - cat /sys/class/net/eth0/mtu
+      - ip -o link show eth0
+      - echo "=== route ==="
+      - ip route
+      - echo "=== resolv.conf ==="
+      - cat /etc/resolv.conf
+      - for i in 1 2 3; do apk add -q --no-cache iputils && break; sleep 5; done || echo "apk failed; falling back to busybox ping (no -M do)"
+      - |
+          echo "=== PMTUD probes (DF=1) to dl-cdn.alpinelinux.org ==="
+          for sz in 1472 1392 1352 1252 1200; do
+            echo "--- payload=$sz (wire ~$((sz+28))) ---"
+            ping -c 2 -W 3 -M do -s $sz dl-cdn.alpinelinux.org || echo "(size=$sz failed)"
+          done
+      - ping -c 3 -W 5 dl-cdn.alpinelinux.org || true
+      - wget --tries=1 --timeout=10 -O /dev/null -nv https://dl-cdn.alpinelinux.org/ 2>&1 || true
+    backend_options:
+      kubernetes:
+        nodeSelector:
+          kubernetes.io/arch: "${ARCH}"
+
+
   - name: Get latest R version
     image: reg.devxy.io/docker.io/library/alpine:3.23
     commands: