Simplify docker image in several ways (#523)

- `--embeddded` mode is the now the default
- embedded cache and ports is also the default
- emsdk now fully polulates the cache by default so no need to do that.
- always "activate" the SDK avoiding that need things to work without
  that.
- avoid making extra symlinks.
- remove python2

docker/test_dockerimage.sh

@@ -8,8 +8,6 @@ node --version
 npm --version
 python3 --version
 pip3 --version
-python --version
-pip --version
 em++ --version
 emcc --version
 java -version