Docker fixes and updates (#461)

Dockerfile:
- Use the most recent stable debian (buster) instead of the previous one
- Build version 1.39.11 by default instead of 1.38.43
- Fix small typo in the comments

Makefile and README.MD: Fix incorrect name of image on dockerhub.

docker/Dockerfile

diff --git a/docker/Dockerfile b/docker/Dockerfile
index 281f731..a1a5363 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -1,9 +1,9 @@
-FROM debian:stretch AS stage_build
+FROM debian:buster AS stage_build
 
 # ------------------------------------------------------------------------------
 
-# Supports only 1.38.40+, accpets also '-upstream' variants
-ARG EMSCRIPTEN_VERSION=1.38.43
+# Supports only 1.38.40+, accepts also '-upstream' variants
+ARG EMSCRIPTEN_VERSION=1.39.11
 ARG EMSDK_CHANGESET=master
 
 # ------------------------------------------------------------------------------

docker/Makefile

diff --git a/docker/Makefile b/docker/Makefile
index a3b16b1..9527fe8 100644
--- a/docker/Makefile
+++ b/docker/Makefile
@@ -11,11 +11,11 @@
 endif
 
 build: .TEST
-	docker build --build-arg=EMSCRIPTEN_VERSION=${version} --tag emscripten/emscripten:${version} .
+	docker build --build-arg=EMSCRIPTEN_VERSION=${version} --tag emscripten/emsdk:${version} .
 
 push: .TEST
-	docker push emscripten/emscripten:${version}
+	docker push emscripten/emsdk:${version}
 ifdef alias
-	docker tag emscripten/emscripten:${version} emscripten/emscripten:${alias}
-	docker push emscripten/emscripten:${alias}
+	docker tag emscripten/emsdk:${version} emscripten/emsdk:${alias}
+	docker push emscripten/emsdk:${alias}
 endif

docker/README.md

diff --git a/docker/README.md b/docker/README.md
index d006dc0..5a72b83 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -3,7 +3,7 @@
 This Dockerfile builds a self-contained version of emsdk that enables emscripten to be used without any
 other installation on the host system.
 
-It is published at https://hub.docker.com/u/emscripten/emscripten
+It is published at https://hub.docker.com/u/emscripten/emsdk
 
 ### Usage
 
@@ -25,7 +25,7 @@
   --rm \
   -v $(pwd):$(pwd) \
   -u $(id -u):$(id -g) \
-  emscripten/emscripten \
+  emscripten/emsdk \
   emcc helloworld.cpp -o helloworld.js
 
 # execute on host machine
@@ -40,7 +40,7 @@
 |`--rm`|remove a container after execution (optimization)|
 |`-v $(pwd):$(pwd)`|Mounting current folder from the host system into mirrored path on the container<br>TIP: This helps to investigate possible problem as we preserve exactly the same paths like in host. In such case modern editors (like Sublime, Atom, VS Code) let us to CTRL+Click on a problematic file |
 |`-u $(id -u):$(id -g)`| Run the container as a non-root user with the same UID and GID as local user. Hence all files produced by this are accessible to non-root users|
-|`emscripten/emscripten`|Get the latest tag of this container|
+|`emscripten/emsdk`|Get the latest tag of this container|
 |`emcc helloworld.cpp -o helloworld.js`|Execute `emcc` command with following arguments inside container, effectively compile our source code|
 
 
@@ -60,7 +60,7 @@
 # using docker
 docker build \
     --build-arg=EMSCRIPTEN_VERSION=1.38.43-upstream \
-    --tag emscripten/emscripten:1.38.43-upstream \
+    --tag emscripten/emsdk:1.38.43-upstream \
     .
 ```
 ```bash
@@ -78,7 +78,7 @@
 
 ```bash
 # using docker
-docker push emscripten/emscripten:1.38.43-upstream
+docker push emscripten/emsdk:1.38.43-upstream
 ```
 ```bash
 # using predefined make target
@@ -90,12 +90,12 @@
 # using docker cli
 
 # in case of fastcomp variant (default backend)
-docker tag emscripten/emscripten:1.38.43 emscripten/emscripten:latest
-docker push emscripten/emscripten:latest
+docker tag emscripten/emsdk:1.38.43 emscripten/emsdk:latest
+docker push emscripten/emsdk:latest
 
 # in case of upstream variant
-docker tag emscripten/emscripten:1.38.43-upstream emscripten/emscripten:latest-upstream
-docker push emscripten/emscripten:latest-upstream
+docker tag emscripten/emsdk:1.38.43-upstream emscripten/emsdk:latest-upstream
+docker push emscripten/emsdk:latest-upstream
 
 ```
 
@@ -116,7 +116,7 @@
 1. create own Dockerfile that holds:
     ```dockerfile
     # Point at any base image that you find suitable to extend.
-    FROM emscripten/emscripten:1.38.25
+    FROM emscripten/emsdk:1.38.25
 
     # Install required tools that are useful for your project i.e. ninja-build
     RUN apt update && apt install -y ninja-build