Enable configcheck lint step to be run in docker

[c-w]

Description

Currently the configcheck lint step can't be run in the Celery Docker container (see screenshot of error message below) which makes it harder to easily verify all the lint steps locally.

This pull request makes it possible to run the configcheck lint step with one Docker command:

docker-compose run -e TOXENV=configcheck --rm celery tox

[codecov]

Codecov Report

Merging #4687 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4687   +/-   ##
=======================================
  Coverage   82.52%   82.52%           
=======================================
  Files         140      140           
  Lines       15894    15894           
  Branches     1980     1980           
=======================================
  Hits        13116    13116           
  Misses       2586     2586           
  Partials      192      192

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[georgepsarakis]

@c-w thank you for your contribution!