Remove redundant syntax

[chihchenghuang]

little things.
I noticed you used virtual package in the alpine dockerfile.
As I know, once you use this feature, you don't have to list all packages in the apk del command.
apk del .build-deps should work perfect here.

https://github.com/tiangolo/uvicorn-gunicorn-docker/blob/master/docker-images/python3.7-alpine3.8.dockerfile#L7

more discussions:
https://stackoverflow.com/questions/46221063/what-is-build-deps-for-apk-add-virtual-command/46222036

Inpired a lot by your repo. good job man!