message: OpenCV.js docker container build instructions
  backport of commit:
diff: @@ -7,6 +7,9 @@ Installing Emscripten
  [Emscripten](https://github.com/kripken/emscripten) is an LLVM-to-JavaScript compiler. We will use Emscripten to build OpenCV.js.
  +@note
  +While this describes installation of required tools from scratch, there's a section below also describing an alternative procedure to perform the same build using docker containers which is often easier.
  +
  To Install Emscripten, follow instructions of [Emscripten SDK](https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html).
  For example:
  @@ -103,3 +106,25 @@ Building OpenCV.js from Source
  @note
  It requires `node` installed in your development environment.
  +
  +Building OpenCV.js with Docker
  +---------------------------------------
  +
  +Alternatively, the same build can be can be accomplished using [docker](https://www.docker.com/) containers which is often easier and more reliable, particularly in non linux systems. You only need to install [docker](https://www.docker.com/) on your system and use a popular container that provides a clean well tested environment for emscripten builds like this, that already has latest versions of all the necessary tools installed.
  +
  +So, make sure [docker](https://www.docker.com/) is installed in your system and running. The following shell script should work in linux and MacOS:
  +
  +@code{.bash}
  +git clone https://github.com/opencv/opencv.git
  +cd opencv
  +docker run --rm --workdir /code -v "$PWD":/code "trzeci/emscripten:latest" python ./platforms/js/build_js.py build_js
  +@endcode
  +
  +In Windows use the following PowerShell command:
  +
  +@code{.bash}
  +docker run --rm --workdir /code -v "$(get-location):/code" "trzeci/emscripten:latest" python ./platforms/js/build_js.py build_js
  +@endcode
  +
  +@note
  +The example uses latest version of [trzeci/emscripten](https://hub.docker.com/r/trzeci/emscripten) docker container. At this time, the latest version works fine and is `trzeci/emscripten:sdk-tag-1.38.32-64bit`
  