v4.0.0 Gluetun

[qdm12]

There is an issue tracking v4.0.0 needed breaking changes: #1113

It's mostly maintenance tasks to throw out old retro-compatible code, which would make mostly my life easier going forward. Nothing major, but a few small environment variables might stop working such as FIREWALL_DEBUG.

[frepke]

For those who want to stick with their current version, simply add pull_policy: never to the compose file (only for Docker Compose).

[jgeorgeson]

Under standard semver rules, breaking changes are expected when going from 3.x.y to 4.x.y.