Configure Docker service image with environment variables in Runners

At the moment, a Docker service can be configured in the bitbucket-pipelines.yml file of each repo, including a custom dind image for Runners and memory.

It has been requested to allow such a configuration of the Docker service image and memory in Linux Docker Runners with environment variables, that will be provided to the docker run command that starts the runner. This would eliminate the need to configure a custom dind image and memory in the bitbucket-pipelines.yml file.