GitLab: Add basic CI (build stage)

.gitlab-ci.yml

+image: python:3.6
+
+stages:
+ - build
+
+before_script:
+  - "pip install -U platformio"
+  - "cp ampel-firmware/config.public.h ampel-firmware/config.h"
+
+job:
+  stage: build
+  script: "platformio run"