(#51) Docs: add a mention of a new Linux environment

Author: ForNeVeR

README.md

@@ -4,7 +4,8 @@ tdlib.native
 This is a project to pack [TDLib][tdlib] (the Telegram Database library) binaries built for several platforms:
 
 - [MacOS 10.15][spec.macos] (x64) _(GitHub Actions image: `macos-10.15`)_
-- [Ubuntu 18.04.4][spec.linux] (x64) _(Appveyor image: `ubuntu`)_
+- [Ubuntu 18.04.4][spec.linux.appveyor] (x64) _(Appveyor image: `ubuntu`)_
+- [Ubuntu 18.04][spec.linux] (x64) _(Docker image: `ubuntu:18.04`)_
 - [Windows Server 2019][spec.windows] (x64) _(Appveyor image: `Visual Studio 2019`)_
 
 Getting Started
@@ -23,7 +24,8 @@ Documentation
 [docs.maintainership]: ./MAINTAINERSHIP.md
 [nuget]: https://www.nuget.org/packages/tdlib.native/
 [releases]: releases
-[spec.linux]: https://www.appveyor.com/docs/linux-images-software/
+[spec.linux.appveyor]: https://www.appveyor.com/docs/linux-images-software/
+[spec.linux]: https://hub.docker.com/_/ubuntu
 [spec.macos]: https://github.com/actions/virtual-environments/blob/main/images/macos/macos-10.15-Readme.md
 [spec.windows]: https://www.appveyor.com/docs/windows-images-software/
 [tdlib]: https://github.com/tdlib/td