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.
nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
