add vlc.js build dockerfile

this will set the environment and sdk to run vlc.js
build script

MR !67

next steps would be :
- source $EMSCRIPTEN_SDK/emsdk_env.sh
- git clone git@code.videolan.org:b1ue/vlc.git --branch vlcjs/wip/upstream.ci.2
- /bin/bash vlc/extras/package/wasm-emscripten/build.sh
- emrun --no_browser vlc.js/vlc.html
1 job for master in 3 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #460512
build

00:03:58