Bug: Update Python inside Dockerfile.build

[wasertech]

I know, I know, it's not really made for us to play with it but it's useful to build the binaries sometimes. Unfortunately, we switched to python 3.8 inside Dockerfile.train but Dockerfile.build still uses python 3.6 which is EOL at this point.

STT/Dockerfile.build

Lines 95 to 96 in 15bef27

	ENV PYTHON_BIN_PATH /usr/bin/python3.6
	ENV PYTHON_LIB_PATH /usr/local/lib/python3.6/dist-packages

[HarikalarKutusu]

I'm with you, it is a real pain to handle those dependencies. Python 3.6 -> (3.7) -> 3.8 transition should be done everywhere.
Build process will be one time and I think it is needed.