Added instructions describing how to build LibVLC in a docker container

Current instructions for building LibVLC were not clear and mentioned docker container failed to build.

Updated instructions using UWP docker image from CI that should work all the time.

README.md

@@ -52,7 +52,19 @@ If you want to make a release build, replace the `Debug` folder name by `Release
 If you want to build libvlc yourself (instead of grabing a nightly build), aside from a lot of time and patience, you need to install and setup `mingw-w64`.
 Then run `./compile.sh` with your target platform and configuration.
 
-Use the docker image [here](https://code.videolan.org/videolan/docker-images/blob/master/vlc-winrt-x86_64/Dockerfile)
+Or you can build LibVLC in a docker container that has environment pre-configured:
+
+- Find the current docker image name that is used on CI for UWP - look for VLC_UWP_LLVM_IMAGE variable in 
+[gitlab-ci.yml](https://code.videolan.org/videolan/vlc-3.0/-/blob/master/extras/ci/gitlab-ci.yml) file.
+
+- Start docker container using that image and one of the folders to host machine. From git shell you can do this as:
+```cmd 
+winpty docker run -it --rm --net=host -v /${PWD}:/vlc registry.videolan.org/vlc-debian-llvm-uwp:20200706065223
+```
+- In your docker container clone repo as described above. If you clone on host Windows machine,
+you'll likely have CRLF line endings messed up.
+- cd to `/vlc/vlc-winrt/libvlc` and run `./compile.sh x86_64 win10` or using different architecture.
+- Compiled libraries will be dropped under one of `vlc-winrt/libvlc/vlc/winrt-...` folders depending on the architecture.
 
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