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/Makefile

@@ -0,0 +1,21 @@
+#!/usr/bin/env make
+
+# Emscripten version to build: Should match the version that has been already released.
+# i.e.:  1.38.45, 1.38.45-upstream
+version =
+alias =
+
+.TEST:
+ifndef version
+	$(error argument 'version' is not set. Please call `make version=SOME_VERSION ...`)
+endif
+
+build: .TEST
+	docker build --build-arg=EMSCRIPTEN_VERSION=${version} --tag emscripten/emscripten:${version} .
+
+push: .TEST
+	docker push emscripten/emscripten:${version}
+ifdef alias
+	docker tag emscripten/emscripten:${version} emscripten/emscripten:${alias}
+	docker push emscripten/emscripten:${alias}
+endif