Improve setup (#123)

* declare sentry volumes external

* move SENTRY_SECRET_KEY to .env file (not in git repo)

* ignore docker-compose.override.yml to allow extending

docker-compose.yml

@@ -16,11 +16,8 @@ x-defaults: &defaults
     - postgres
     - memcached
     - smtp
-
+  env_file: .env
   environment:
-    # Run `docker-compose run web config generate-secret-key`
-    # to get the SENTRY_SECRET_KEY value.
-    SENTRY_SECRET_KEY: ''
     SENTRY_MEMCACHED_HOST: memcached
     SENTRY_REDIS_HOST: redis
     SENTRY_POSTGRES_HOST: postgres
@@ -64,4 +61,6 @@ services:
 
 volumes:
     sentry-data:
+      external: true
     sentry-postgres:
+      external: true