Add 2nd Dockerfile for port 8000

[swallez]

Prepare a dedicated Docker file for environments that need a default configuration starting in http mode on port 8000:

• revert Change default port to 8000 #142 and set back the default port to 8080
• allow http listen address to be set using the HTTP_ADDRESS env var
• add a new Dockerfile-8000 docker build that defaults HTTP_ADDRESS to 0.0.0.0:8000 and sets the default CLI arguments to http (no need to use the CLI_ARGS workaround)
• bump version to 0.4.3