Add build stage for continuous integration

.gitlab-ci.yml:
Add a build stage to the gitlab CI, that facilitates the scripts below .gitlab/ci/, building the baseline and releng
profiles in parallel.
Distinguish the use-case in which builds are done for master, schedules and tags in a secure environment and any other
where builds just have to be fast (for ensuring nothing is broken).

Use MiB as block size for the du call when generating data for the metrics file.

.gitlab-ci.yml

@@ -3,11 +3,52 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 default:
-  image: archlinux:latest
-  before_script:
-    - pacman --noconfirm -Syu --needed base-devel shellcheck
+  image: "archlinux:latest"
 
-lint:
-  stage: build
+stages:
+  - lint
+  - build
+
+shellcheck:
+  stage: lint
+  before_script:
+    - pacman --noconfirm -Syu --needed make shellcheck
   script:
     - make lint
+
+.build:
+  stage: build
+  before_script:
+    - pacman -Syu --needed --noconfirm qemu-headless libisoburn
+  script:
+    - ./.gitlab/ci/build-host.sh
+  after_script:
+    - echo "image_size_megabytes{image=\"${PROFILE}\"} $(du --block-size=MiB -- output/${PROFILE}/*iso)" > metrics.txt
+  parallel:
+    matrix:
+      - PROFILE: [baseline, releng]
+  artifacts:
+    name: "output"
+    paths:
+      - "output/*/*"
+    expire_in: 2d
+    reports:
+      metrics: metrics.txt
+
+build:
+  extends: .build
+  tags:
+    - fast-single-thread
+  except:
+    - master
+    - schedules
+    - tags
+
+build:secure:
+  extends: .build
+  tags:
+    - secure
+  only:
+    - master
+    - schedules
+    - tags