Provided host and port informations for use within Docker container.

With this modification I can run Spring Boot Admin within a Docker container. I provide host and port of my Docker's host as environment variables.

An example of Docker Run command: 

docker run -d --name myservice -p 8091:8081 -e server.port=8081 -e spring.boot.admin.url='http://192.168.10.139:8091' -e spring.boot.admin.host='192.168.10.139' -e spring.boot.admin.port='8091' myservice:1.0.0

Hope you enjoy...