[Tests] reduce CI flakiness from transient Docker registry failures

Container-based suites and the `nvm_download` httpbin check hard-fail whenever DockerHub is briefly unreachable
(observed: `dial tcp ...:443: connect: connection refused` while pulling images),
even though the change under test is fine.
This is unrelated to any test logic.

- tests-xenial / tests-installation-node: retry the `docker pull` up to 5 times before `docker run`, mirroring the existing apt-get retry
- `nvm_download` test: retry the httpbin pull and skip the auth-header checks (rather than fail) when the image cannot be pulled or run, and make cleanup tolerant of a missing container.

.github/workflows/tests-installation-node.yml

@@ -55,6 +55,12 @@ jobs:
       - run: npx which urchin
       - name: Run installation_node tests in container
         run: |
+          # Retry the image pull to tolerate transient Docker registry failures
+          for i in 1 2 3 4 5; do
+            docker pull ubuntu:16.04 && break
+            echo "docker pull failed, attempt $i/5"
+            sleep $((i * 5))
+          done
           docker run --rm \
             -v "${{ github.workspace }}:/workspace" \
             -w /workspace \