Remove SSH_AUTH references in docker compose

It was added in commit 1db5a00ea, probably due to the Capistrano
configuration of the developer who wrote the code. On my machine, docker
compose crashed due to these lines.

docker-compose.yml

@@ -34,10 +34,8 @@ services:
     volumes:
       - .:/var/www/consul
       - bundle:/usr/local/bundle
-      - "$SSH_AUTH_SOCK:/tmp/agent.sock"
     environment:
       - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
-      - SSH_AUTH_SOCK=/tmp/agent.sock
 volumes:
   db_data: {}
   bundle: {}