Add regular CI tests for exaslct on Centos

[tkilias]

Situation:

• Currently, we test exaslct only on Ubuntu, but some users want to use it under Centos 7 or 8

Tasks:

• Improve and extend https://github.com/exasol/script-languages/blob/1e0cac875a71d8da757ea14218a524454b1af3b6/test_exaslct_init_script.sh to allow multiple Guest OSes for the surrounding Docker Container
  • Maybe integrate it in the usual python unittests
• Add docker file for Centos with python 3.6
• Add different test scenarios
  • exaslct --help
  • exaslct build
  • exaslct export

[tkilias]

Example Dockerfile for Centos7 with Python 3.6

FROM centos/python-36-centos7
  
USER root // the Docker base image sets the USER to 1001 without root rights, but to access mounted volumes we often need root
RUN python3 -m pip  install virtualenv
RUN python3 -m virtualenv --python=python3 venv

RUN mkdir test
COPY . /test
#RUN chown -R 1001:1001 /test
RUN rm /test/Pipfile.lock || true

#USER 1001

[tkilias]

Fixed in exasol/script-languages-container-tool#51