add basic CI capabilities

.gitlab-ci.yml

+image: $CONTAINER_TEST_IMAGE
+
+variables:
+  CONTAINER_TEST_IMAGE: gitlab-registry.mpcdf.mpg.de/$CI_PROJECT_PATH:$CI_BUILD_REF_NAME
+  OMP_NUM_THREADS: 2
+
+stages:
+  - build_docker
+  - testing
+
+build_docker:
+  image: docker:stable
+  stage: build_docker
+  script:
+    - docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN gitlab-registry.mpcdf.mpg.de
+    - docker build -t $CONTAINER_TEST_IMAGE .
+    - docker push $CONTAINER_TEST_IMAGE
+
+test_suite:
+  stage: testing
+  script:
+    - cd pypocketfft
+    - pip3 install --user .
+    - pytest-3 -q test.py

Dockerfile

+FROM debian:testing-slim
+RUN apt-get update && apt-get install -y git python3-pip python3-pytest && rm -rf /var/lib/apt/lists/*