Updated docker configuration examples and instructions

[AMufInABox]

Since some people have been having some trouble with setting up the bot with Docker, I thought I'd update the instructions for it along with some of the configuration files so that they're more up to date with the repo.

• Fixed bot.image to point back to gnomeddev/discord-tts-bot as Discord-TTS/Bot doesn't exist
• Removed network_mode: "host" from bot and tts-service as it broke the DNS resolution when starting the bot
• Added a healthcheck to database and switched the bot's depends_on to wait on the database to be healthy and for tts-service to fully start
• Updated the gcp.json mount so that it just points to ./gcp.json directly

I'm not exactly sure why this previously mounted to ${GOOGLE_APPLICATION_CREDENTIALS}? seems simpler to just mount the file directly

• Added AWS credential env variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION) to tts-service and created a new .env.example file due to Polly voices being required by tts-service
• Updated README's "Normal (Docker)" section
  • docker-compose -> docker compose (since docker composes is mainly v2 now)
  • Added steps for filling in .env and placing the GCP service key as gcp.json
  • Added a note that both are required for the bot to start (due to tts-service requiring it)