adds dhcpv6 support via extra tag

[ticevi]

adds new v6 tag to run dhcpv6 server in Docker
#20

[robinsmidsrod]

@ticevi I would suggest implementing this using an environment variable instead, so that the same entrypoint can run either IPv4 (default) or IPv6. Then you won't have so much duplication of code, following the DRY principle.

[ticevi]

@robinsmidsrod good point. I have adapted my merge request

[robinsmidsrod]

@ticevi Good direction! Can you change it even further, so that it uses -e DHCPD_PROTOCOL=6 (with value 4 being the default) to make the choice, and just have a single run line in entrypoint so that this environment variable generates a -4 or -6 argument to the final run command?