add gitlab CI pipeline

The current pipeline (from Mobian) just doesn't work, we should get to
a more efficient, simpler one.

With the `clang-format` config now in place, this pipeline has 2 jobs:
* ensure the code is properly formatted
* run a test build to make sure it won't break

.gitlab-ci.yml

@@ -0,0 +1,39 @@
+stages:
+  - build
+  - check
+
+variables:
+  CLANG_VERSION: 19
+  BUILD_DEPS_NO_MM: "build-essential libcurl4-openssl-dev libgpiod-dev libgudev-1.0-dev libusb-1.0-0-dev meson scdoc"
+  BUILD_DEPS: "${BUILD_DEPS_NO_MM} libmm-glib-dev"
+
+image: debian:unstable-slim
+
+build:
+  stage: build
+  script:
+    - apt-get update
+    - apt-get -y install ${BUILD_DEPS}
+    - meson build
+    - meson compile -C build
+  artifacts:
+    paths:
+      - build
+
+# ModemManager is an optional dependency, let's ensure eg25-manager still builds fine without it
+build-no-mm:
+  stage: build
+  script:
+    - apt-get update
+    - apt-get -y install ${BUILD_DEPS_NO_MM}
+    - meson build
+    - meson compile -C build
+
+format:
+  stage: check
+  dependencies:
+    - build
+  script:
+    - apt-get update
+    - apt-get -y install ${BUILD_DEPS} clang-format-${CLANG_VERSION}
+    - ninja -C build clang-format-check