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.

https://github.com/coqui-ai/STT/blob/15bef2788a0607c78b41d14fb00f5eb5b9cd55d7/Dockerfile.build#L95-L96