Added docker service for PowerDNS 4.4

[frankvanhest]

Extended the docker-compose.yml for PowerDNS 4.4

Description

The docker hub https://hub.docker.com/r/psitrax/powerdns does not have a tag for PowerDNS 4.4, but the tag latest has the correct version.
The pdns.conf is different in 4.4. The fields default-soa-name and default-soa-email are removed and replaced by default-soa-content.

It is best to use the tag 4.4 when it becomes available. For now this will at least prove that the code works for PowerDNS 4.4.

Motivation and context

I would love to have support for PowerDNS 4.4

fixes #66

How has this been tested?

By running run-tests.sh

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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[trizz]

@frankvanhest We've just tagged a release with your changes. Thanks!