Update Dockerfile to pull from S3 endpoint in EU, not from servers in China

[oskapt]

The docs list an S3 endpoint for downloading the source, but the Dockerfile instead pulls from a server in China.

While both of these have the same MD5 signature today, content stored in China is subject to regulation and laws that easily open Seafile up to a supply chain attack for anyone using the Docker container image. It would be trivial to change the source of the version in China and have that shipped out across the world.

For a product that manages files and data for individuals and organizations, this would be a disaster.

Can the Dockerfile pull from the same endpoint as the docs? Extra credit if you publish the MD5 signature in the docs and validate it from a public HTTP endpoint (e.g. https://seafile.com/hash/8.0.7) during the Docker build.

[GogoFC]

I'm pretty sure many of the developers live in China :) , no offence just a little fun. Seafile isn't Huawei :)