CI: use Alpine instead of Debian for py-sh-static job

Shellcheck made its way into Alpine, so we can now use Alpine instead of Debian in .gitlab-ci.yml:

# static code analysis (shellcheck is not in Alpine, so we use Debian)
py-sh-static:
  stage: first
  <<: *only-default
  image: python:3.6-slim-stretch
  before_script:
    - apt -q update >/dev/null
    - apt -y install flake8 shellcheck >/dev/null
  script:
    - .gitlab-ci/static_code_analysis.sh

This one is rather trivial, who wants to take it?