No email settings, only "Arguments -i"

The issue is because of this line

## Plugins go here
env:
  SEGMENT_IO_KEY: XXXXXXX

You are redefining the “env” key (which is defined earlier) and thus all of the “DISCOURSE_” settings are gone…
To fix it, you need to move the “SEGMENT_IO_KEY” under the “#DISCOURSE_CDN_URL” line, like so

env:
  LANG: en_US.UTF-8
  # DISCOURSE_DEFAULT_LOCALE: en

  # [...]

  ## The CDN address for this Discourse instance (configured to pull)
  #DISCOURSE_CDN_URL: [REMOVED]

  SEGMENT_IO_KEY: XXXXXXX

## The Docker container is stateless; all data is stored in /shared
volumes:
  - volume:
      host: /var/discourse/shared/standalone
      guest: /shared
  - volume:
      host: /var/discourse/shared/standalone/log/var-log
      guest: /var/log