Without it, DAD is enabled and it takes more than a second for an IPv6
address to be no longer tentative and become usable.

This affects announceIPs(), which fails to send IPv6 NA.

I assume, this also meant that the container already started at
a point when IPv6 is not yet fully usable. For example, binding
to the address would fail.

Instead, enable optimistic_dad, so that we can use the address right
away.

Don't let the first error shortcut the announcement of subsequent
addresses. Loop over all addresses, and return the joined errors. Note
that the caller anyway ignores the error and only uses it for debug
logging.