docs: Add Dockerfile and docker build instructions (#3547)

Many devs using Shaka and wishing to compile the library are required to install all prerequisites on their machine. For docker devs, this is extremely bad as it requires installing Java and Python on OS.
This commit adds a basic configuration for quick compilation via Docker, without requiring any other prerequisites. Improvements can be made in order to allow all Shaka workflow inside Docker.

Suggested in #181

docs/tutorials/welcome.md

@@ -35,6 +35,8 @@ To get the sources and compile the library, you will need:
     * _NOTE: A local web server is required because browsers place restrictions
       on applications from file:/// URLs._
 
+If you just want to compile for export to other projects, you might consider compiling through a docker container. (see compile instructions)
+
 To quickly install these prerequisites on Ubuntu or Debian, you can run this
 script:
 
@@ -62,6 +64,12 @@ cd shaka-player
 python build/all.py
 ```
 
+Alternatively you can use a docker container:
+```sh
+cd build/docker
+docker-compose run --rm shaka-compiler
+```
+
 The output is:
  * dist/shaka-player.compiled.js (compiled bundle)
  * dist/shaka-player.compiled.debug.js (debug bundle)