Docker image docs

[greenled]

Documentation should include how to use the Docker image

[greenled]

Something like

docker run -v $(pdw)/docker-compose.yml:/docker-compose.yml -e ACTION="deploy" -e PORTAINER_USER="admin" -e PORTAINER_PASSWORD="password" -e PORTAINER_URL="http://portainer.example.com" -e PORTAINER_STACK_NAME="mystack" -e DOCKER_COMPOSE_FILE="docker-compose.yml" greenled/portainer-stack-utils psu

docker run -v $(pdw)/docker-compose.yml:/docker-compose.yml greenled/portainer-stack-utils psu -a deploy -u admin -p password -l http://portainer.example.com -n mystack -c docker-compose.yml