Add ipv6 support to docker container

[edameki]

Added functionality for fdb.bash to support ipv6, provided the new env variable FDB_IP_VERSION is set to 6 (or any string containing 6, such as v6, ipv6, iwantipv6, etc. )

There is an unfortunate caveat of no dual stack support, as FDB coordinators are recognized by their IP+port and not their machineid, to the best of my knowledge. If I'm wrong and FDB does not double count a coordinator with both IPv4 and IPv6 addresses in the cluster file, I would be happy to add dual stack support to this.

This was part of now-split-up #13822, with slightly more robust logic.

Changes

IPv6 Support

Issue

Before now, fdb.bash would throw an error if hostname -i resolved to a v6 address, as it would not be encased in brackets.

New Behavior

Depending on the value of FDB_IP_VERSION, fdb.bash will try and find the machine's v4 or v6 address. IPv4 is the default pick.

If hostname -i resolves to the correct IP version, that is used as the public IP (as before.) Otherwise, the first valid ip in hostname -I is used. This will prevent a dual stack container from erroring out if IPv6 is desired, as hostname -i seems to prefer the v4 address to the v6 if both are present.

Otherwise, an error is thrown that no valid IP could be found.

Enhanced FDB_COORDINATOR ip resolution

Issue

FDB Coordinator discovery always looks up IPv4 addresses, as dig by default searches for A records. It also doesn't respect the local /etc/hosts file.

New Behavior

Based on the value of FDB_IP_VERSION, fdb coordinator ip is discovered with either getent ahostsv4 or getent ahostsv6. This will prefer the local hosts file if it has the name, otherwise it will try to reach a remote DNS server.