[FEATURE] Provide docker based development environments

[mschwemer]

The goal is to provide simple to use development environments for EXT:powermail.

There will be two variants:

• a plain docker setup (using https://github.com/codekitchen/dinghy-http-proxy)
• a ddev - based installation

[mschwemer]

There is still an issue with the behavior tests.

They are running, but many tests fail because of changed texts in the labels.

Is this intended?