Docker container can't be used, produces error that python 3.8 can't be found.

### Description

Docker container can't be used, produces error that python 3.8 can't be found.
Reason: Dockerfile contains wrong Python version.

### How To Reproduce

Download linux tar, extract run docker as described:

docker build -t scancode-toolkit .

Result:
> [6/6] RUN ./scancode --reindex-licenses:
#10 0.256 * Configuring ScanCode for first use...
#10 0.266 /scancode-toolkit/configure: line 105: Python3.8: command not found

Changing the first line in the dockerfile from

FROM python:3.6-slim-buster

to

FROM python:3.8-slim-buster

solves the problem.

### System configuration

* Windows with Windows docker client
* 31.0.0b4_py38