Various fixes inspired by work on deploying weakforced in K8S

[neilcook]

• Fix an issue where IPv6 ComboAddress returned zero port number (which caused v6 HTTP listen addresses to not work)
• Update to use more recent cmake, prometheus-cpp and libdrogon releases
• Fix build issues caused by the updates to prometheus and drogon
• Set V6ONLY socket option to stop v6 sockets from managing v4 addresses for replication
• Push an unstable docker image tag to docker-hub from the master branch
• Add net-tools to the wforce docker image image for easier debugging
• Add config option to disable password for /metrics endpoint

[neilcook]

Yes I need to to rationalise this, as the same (or similar) build instructions are split between different dockerfiles (regression, builder), GitHub Actions etc. It's a bit of a mess... another PR methinks...