issue 549 prevent mailman-web dns lookup from throwing

[mm534]

Addresses #549

There might be some neater options than just swallowing the exception, but I've tried to keep the change minimal.

Alternatives -

• DJANGO_ALLOWED_HOSTS could override rather than extend, but that would be a breaking change.
• We could introduce another env var to override the default behavior.

[maxking]

This looks fine, fewer env vars to document are better. DJANGO_ALLOWED_HOSTS already exists, so no need add another one for just specifying the name of the container.

Thanks for your contribution!