add support for arm64 for docker images

[sychevsky]

Please add support for arm64 for docker images

[patrickdk77]

I've added support in my branch if you want to take a look.
https://hub.docker.com/r/patrickdk/s3-proxy

[tillkuhn]

@patrickdk77 Thanks, I see you also added DIRECTORY_LISTINGS_CHECK_INDEX which allows listing and default index.html at the same time, which I consider a very useful feature. Hope you keep maintaining your fork!