Fix docker test

[nsano-rururu]

Fix

Facing issue “virtualenv: error: argument --setuptools: expected one argument” when running jertel/elastalert
https://stackoverflow.com/questions/65020700/facing-issue-virtualenv-error-argument-setuptools-expected-one-argument-w

I've been trying to run the jertel/elastalert repo. The config.yaml and rules.yaml have both been set up to point at our environment. The only other change has been in setup.py where we changed elasticsearch==7.0.0 to 7.6.0

When I'm building the docker container though, I get stonewalled by this error.

[jertel]

Confirmed - this PR fixes the Dockerfile-test build. Thanks for submitting this.