Added dockerfiles form trzeci/emscripten-docker (#368)

As a continuation from: https://github.com/emscripten-core/emscripten/issues/8742

This PR provides adopted Dockerfile from https://github.com/trzecieu/emscripten-docker/blob/master/docker/trzeci/emscripten-fastcomp/Dockerfile which supports builds:
  * fastcomp
  * upstream

Features:
* An image can be build with fastcomp and upstream version of Emscripten SDK
* It applies some extra size optimizations (that might be removed in sake of clarity / stability)
* It creates conventional `1000:1000` user `emscripten`
* It creates extra entrypoint that mitigates running as non-root user (Some context: https://trzeci.eu/fixing-permission-of-files-created-from-docker/)
* It has simple-stupid tests performed during building

docker/test_dockerimage.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+set -ex
+
+which asm2wasm
+which llvm-ar
+which emsdk
+node --version
+npm --version
+python3 --version
+pip3 --version
+python --version
+pip --version
+em++ --version
+emcc --version
+java -version
+cmake --version
+
+# cleanup after test
+find ${EMSDK} -name "*.pyc" -exec rm {} \;