Rename the environment variable for the docker host

[johannesduesing]

Describe the problem
The environment variable used for overriding the default docker host used by the registry is currently named DOCKER_HOST. This name is used by various applications, including some docker functionalities themselves, so we should make the variable name more unique to avoid name clashes.

Describe the solution you would like
Rename the environment variable to DELPHI_DOCKER_HOST.

[johannesduesing]

Closed with #88