CI addition

.gitlab-ci.yml

+image: clarinpl/python:3.6
+
+cache:
+  paths:
+    - .tox
+
+stages:
+  - check_style
+  - build
+
+before_script:
+  - pip install tox==2.9.1
+
+pep8:
+  stage: check_style
+  script:
+    - tox -v -e pep8
+
+docstyle:
+  stage: check_style
+  script:
+    - tox -v -e docstyle
+
+build_image:
+  stage: build
+  image: docker:18.09.7
+  only:
+    - master
+  services:
+    - docker:18.09.7-dind
+  before_script:
+    - ''
+  script:
+    - docker build -t clarinpl/polem .
+    - echo $DOCKER_PASSWORD > pass.txt
+    - cat pass.txt | docker login --username $DOCKER_USERNAME --password-stdin
+    - rm pass.txt
+    - docker push clarinpl/polem