Add ability to build the arm image on a non-arm system

[nvllsvm]

Similar to the Dockerfile.arm64 image, the Dockerfile.arm image should be buildable on x86_64 systems.

This was attempted in 1c82455 but failed with the following error:

  Creating directory "obj/Debug/netstandard2.0/".
Done Building Project "/repo/BDInfo/BDInfo.csproj" (Clean target(s)).
Done Building Project "/repo/MediaBrowser.MediaEncoding/MediaBrowser.MediaEncoding.csproj" (Clean target(s)).
Done Building Project "/repo/Emby.Server.Implementations/Emby.Server.Implementations.csproj" (Clean target(s)).
Done Building Project "/repo/Jellyfin.Server/Jellyfin.Server.csproj" (Clean target(s)).
Project "/repo/MediaBrowser.sln" (1) is building "/repo/ThirdParty/taglib-sharp/src/taglib-sharp.csproj" (25:3) on node 1 (Clean target(s)).
Project "/repo/ThirdParty/taglib-sharp/src/taglib-sharp.csproj" (25:3) is building "/repo/ThirdParty/taglib-sharp/src/taglib-sharp.csproj" (25:5) on node 1 (Clean target(s)).
CoreClean:
  Creating directory "obj/Debug/net40/".
Done Building Project "/repo/ThirdParty/taglib-sharp/src/taglib-sharp.csproj" (Clean target(s)).
Done Building Project "/repo/ThirdParty/taglib-sharp/src/taglib-sharp.csproj" (Clean target(s)).
Done Building Project "/repo/MediaBrowser.sln" (Clean target(s)).

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:19.86
Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

/usr/share/dotnet/sdk/2.2.103/NuGet.targets(114,5): error : Object reference not set to an instance of an object. [/repo/Jellyfin.Server/Jellyfin.Server.csproj]
The command '/bin/sh -c export DOTNET_CLI_TELEMETRY_OPTOUT=1  && dotnet clean -maxcpucount:1  && dotnet publish     -maxcpucount:1     --configuration release     --output /jellyfin     Jellyfin.Server' returned a non-zero code: 1

[jellyfin-bot]

We are moving all feature and enhancement requests to our new Fider platform here. This new platform lets people vote on and better manage such requests.
This request now lives here.