Add example .gitlab-ci.yml from pio docs

Copy and pasted from
https://docs.platformio.org/en/latest/integration/ci/gitlab.html
with a few small changes.

.gitlab-ci.yml

@@ -0,0 +1,20 @@
+image: python:3.9
+
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/.pip-cache"
+  PLATFORMIO_CACHE_DIR: "$CI_PROJECT_DIR/.platformio-cache"
+
+cache:
+  paths:
+    - .pip-cache/
+    - .platformio-cache/
+
+stages:
+ - build
+
+before_script:
+  - "pip install -U platformio"
+
+build_all:
+  stage: build
+  script: "pio run"