Be able to give a `policy.xml` by environment variable

[allanlegalstart]

s3rver let you specificy your cors policy through the --cors option at launch , which is especially useful when you want to let your user to upload document to s3 (through signed url)

it would be great to expose this possibility through an environment variable

[jean553]

Hi! Thanks for the suggestion. Here a PR I have just made: https://github.com/jean553/docker-s3-server-dev/pull/8/files. Let you check and provide me your feedback if it matches your need.