I'm pretty sure it's no big deal to install Portainer on Synology -- I think @chDVRuser said it took him about a minute. I'm pretty confident it would make your life easier.
But, as to your question:
The environment section is wrong in your screenshot. Here's my recommended docker-compose that I posted yesterday:
version: '3.9'
services:
# GitHub home for this project with setup instructions: https://github.com/jgomez177/pluto-for-channels
# Docker container home for this project: https://ghcr.io/jgomez177/pluto-for-channels
pluto-for-channels:
image: ghcr.io/jgomez177/pluto-for-channels:${TAG}
container_name: pluto-for-channels
ports:
- ${HOST_PORT}:${PLUTO_PORT} # Use the same port number the container is using, or optionally change it if the port is already in use on your host
environment:
- PLUTO_PORT=${PLUTO_PORT}
- PLUTO_CODE=${PLUTO_CODE} # ALLOWED_COUNTRY_CODES: us_east - United States East Coast, us_west - United States West Coast, local - Local IP address Geolocation, ca - Canada, uk - United Kingdom,
restart: unless-stopped
# Default Environment variables can be found below under stderr -- copy and paste into Portainer-Stacks Environment variables section in Advanced mode
With these environment variable examples:
TAG=latest
HOST_PORT=7779
PLUTO_PORT=7777
PLUTO_CODE=local,us_west,us_east,ca,uk
You can, of course hard code the env var values into the compose, which would look like this in my example:
version: '3.9'
services:
pluto-for-channels:
image: ghcr.io/jgomez177/pluto-for-channels:latest
container_name: pluto-for-channels
ports:
- 7779:7777 # Use the same port number the container is using, or optionally change it if the port is already in use on your host
environment:
- PLUTO_PORT=7777
- PLUTO_CODE=local,us_west,us_east,ca,uk
restart: unless-stopped
Much more elegant, and easier to manage, using Portainer with env vars though! 
