Support Alpine linux

[natemcmaster]

Requires compiling for musl, not glibc.

Workaround
Users can remove libuv from their app and rely on the system's installed version of libuv instead.

<ItemGroup>
  <PackageReference Include="Libuv" Version="1.10.0" ExcludeAssets="Native" />
</ItemGroup>

Then, install libuv on host machines.

apk add libuv
ln -s /usr/lib/libuv.so.1 /usr/lib/libuv.so

[muratg]

cc @glennc

[Petermarcu]

Can you flip back to using sockets to unblock temporarily?

[natemcmaster]

At the moment, Kestrel's default is still sockets in 2.1 preview1 bits. This default is subject to change before RTM. aspnet/KestrelHttpServer#2220

[Petermarcu]

Is see, I haven't seen the error for Alpine but what is blocking then? Is it that tests are failing on Alpine because they are explicitly running on libuv?

[natemcmaster]

Right. When I say it's a blocker, I mean blocking moving forward with Alpine in a supported way. We can disable tests, and produce a shared framework is broken for several key scenarios on Alpine...but I wouldn't want to ship that.

[Petermarcu]

We need to unblock product construction first. Alpine is already noted as a best effort OS and not in our supported list. Could you just disable the tests on Alpine? I also thought tests were not critical path for product construction

[natemcmaster]

Yes we can do those things. I updated my PR to the CLI last night to skip tests and pull our portable sharedfx even though it won't work. dotnet/cli#8341

[Petermarcu]

Sounds good. Thanks! Hopefully Alpine will be off the blocking list then.

[natemcmaster]

Punting to backlog as we want to make sockets the default transport in Kestrel. Users could workaround for now by installing the libuv apk.