Fedora builds were broken due to dotnet-sdk-3.1 and dotnet-runtime-3.1 not being available on the @dotnet-sig/dotnet repo as well as a lack of the git package (needed for yarn when installing jellyfin/jellyfin-noto per https://github.com/jellyfin/jellyfin-web/pull/735). Replaces the outdated repo with the official repo found in Microsoft's documentation.
This performs a lot of bugfixing and general cleanup to the
Fedora/CentOS builds, including moving the create_tarball into the
docker-build.sh script, remove some old long versions from the spec
file, correcting several bugs with the Docker environment including
splitting them into more discrete layers, and finally making sure
jellyfin-web is included properly in the RPM.
