A fix for countTokens and a dockerfile improvement

[filiptrplan]

Without this fix I got a 500 error when trying to upload images because the input would be an array that contained the image and the text.

I also added a Dockerfile argument to use it with docker compose. Example usage:

  copilot:
    build:
      context: ./copilot-api
      dockerfile: Dockerfile
      args:
        GH_TOKEN: ${GH_TOKEN}
    container_name: copilot
    env_file:
      - .env
    ports:
      - "4141:4141"

[ericc-ch]

looks good, thank you!
also I think this should close #27