Add docker override file to docs and gitignore

The docker-compose.yml file is likely to be edited quite extensively by
admins when setting up an instance. This would likely cause problems
when dealing with updating Akkoma as merge conflicts would likely occur.

Docker-compose already has the ability to use override files in addition
to the main `docker-compose.yml` file. Admins can instead put any
overrides (additional volumes, container for elasticsearch, etc.) into a
file that won't be tracked by git and thus won't run into merge
conflicts in the future. In particular, the
`docker-compose.override.yml` will be checked by docker compose in
addition to the main file if it exists and override definitions from the
latter with the former.

docker-compose.yml

@@ -45,7 +45,7 @@ services:
     volumes:
       - .:/opt/akkoma
 
-  # Uncomment the following if you want to use a reverse proxy
+  # Copy this into docker-compose.override.yml and uncomment there if you want to use a reverse proxy
   #proxy:
   #  image: caddy:2-alpine
   #  restart: unless-stopped