Docker "label" functionality to limit containers seen in Pangolin UI

[JonnyBooker]

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Description

I wanted the ability to be able to limit which containers pangolin can see. I have containers locally that I would not want to expose and seeing them in a list amongst containers that I did felt redundant and I didn't want to give the option in the UI.

Similar to traefik/watchtower, I have added functionality where if a new environment variable is set DOCKER_LABEL_ENABLE to the value of true, that any container without a dedicated new label of newt.docker.enable=true, would be ignored and not sent to pangolin. Default not setting of this variable would keep existing behaviour.

This does not disable access to the container itself, more disables it from being shown in pangolin, it could still be connected to if you knew the IP address. Therefore if a different name would be preferred, then that would be fine. I was following in the footsteps of other apps. Could have the potential that could call list containers once more to get the enabled one, matching up the IP address of the target against docker containers that are label enabled and reject connecting up this server? Open to suggestions on that one if should that.

I have also made it output the values of the variables being used if the log level is lowered to DEBUG so you can verify you are seeing the values you expect.

I haven't got any experience before today in Go so any conventions/formatting/aspects you would like me to change, I'm completely open. I just wanted to see if could make it happen and offer the option if people wanted it.

How to test?

1. A build of newt would be required from this branch, in this instance I have built the dockerfile locally in the root git repository using:

docker build . -t newt:test

2. Using docker compose below, a valid newt id/secret/endpoint will be required
   Here is an example docker-compose.yml

services:
  newt:
    image: newt:test
    container_name: newt
    restart: unless-stopped
    environment:
      - PANGOLIN_ENDPOINT=<Endpoint Value>
      - NEWT_ID=<ID Value>
      - NEWT_SECRET=<Secret Value>
      - DOCKER_LABEL_ENABLE=true
      - DOCKER_SOCKET=/var/run/docker.sock
    networks:
      - newt-network
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

  nginx-example-1:
    image: nginx:latest
    container_name: nginx-example-1
    networks:
      - newt-network
    restart: unless-stopped

  nginx-example-2:
    image: nginx:latest
    labels:
      - newt.docker.enable=true
    container_name: nginx-example-2
    networks:
      - newt-network
    restart: unless-stopped

networks:
  newt-network:

In pangolin, only one container should be visible via the dashboard:

Logs will be visible if using a debug log level saying what is and isn't included:

Output of the debug logs for env variables:

[JonnyBooker]

Apologies, targetting the branch incorrectly to begin with, updated now

[JonnyBooker]

Closing this PR after discussion on discord in favour of other functionality to control container access in Pangolin