[centos8]python does not exist when run repo init

[hqbai]

Brief Description
-----------------
'python' does not exist when run 'repo init' in the build docker with f/centos8 branch.

Severity
--------
Minor

Steps to Reproduce
------------------
cd $HOME
git clone https://opendev.org/starlingx/tools.git
git checkout f/centos8

After created the build docker successfully, enter the docker and run:
[user@e2937a7eac6c ct8stx]$ repo init -u https://opendev.org/starlingx/manifest -m default.xml -b f/centos8
env: ‘python’: No such file or directory

Expected Behavior
------------------
"repo init" can be executed
Actual Behavior
----------------
"repo init" can not be done for no 'python'

Reproducibility
---------------
Always

System Configuration
--------------------
Designer environment, checkout f/centos8 of starlingx/config

Branch/Pull Time/Commit
-----------------------
f/centos8

Last Pass
---------
any tools branch with python2.7 is ok, after python3.6 is ported, this issue happens.

Timestamp/Logs
--------------
[user@e2937a7eac6c ct8stx]$ repo init -u https://opendev.org/starlingx/manifest -m default.xml -b f/centos8
env: ‘python’: No such file or directory

Test Activity
-------------
Developer Testing

Workaround
----------
in docker shell:
ln -sfn /usr/bin/python3 /usr/bin/python