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

.gitignore

@@ -65,5 +65,11 @@ target/
 # pyenv
 .python-version
 
+# https://docs.docker.com/compose/extends/
+docker-compose.override.yml
+
+# env config
+.env
+
 *.tar
 data/