gitlab CI: add a build job for Ubuntu 20.04

To avoid adding dependencies that may be considered a bit too recent, let's
build on the current Ubuntu LTS.

Unlike the current Fedora job, this is just the default build with no special
options. They can be added by anyone motivated enough to track down the
required package names :)

.gitlab-ci.yml

@@ -15,6 +15,9 @@ include:
   - project: 'freedesktop/ci-templates'
     ref: *templates_sha
     file: '/templates/fedora.yml'
+  - project: 'freedesktop/ci-templates'
+    ref: *templates_sha
+    file: '/templates/ubuntu.yml'
 
 .fedora:
   variables:
@@ -49,6 +52,34 @@ include:
       which
       xmltoman
 
+.ubuntu:
+  variables:
+    # Update this tag when you want to trigger a rebuild
+    FDO_DISTRIBUTION_TAG: '2021-06-10.0'
+    FDO_DISTRIBUTION_VERSION: '20.04'
+    FDO_DISTRIBUTION_PACKAGES: >-
+      debhelper-compat
+      findutils
+      git
+      libasound2-dev
+      libavcodec-dev
+      libavfilter-dev
+      libavformat-dev
+      libdbus-1-dev
+      libglib2.0-dev
+      libgstreamer1.0-dev
+      libgstreamer-plugins-base1.0-dev
+      libsbc-dev
+      libsdl2-dev
+      libudev-dev
+      libva-dev
+      libv4l-dev
+      libx11-dev
+      meson
+      pkg-config
+      systemd
+      xmltoman
+
 .coverity:
   variables:
     FDO_REPO_SUFFIX: 'coverity'
@@ -92,6 +123,14 @@ include:
     paths:
       - build-*/meson-logs
 
+container_ubuntu:
+  extends:
+    - .ubuntu
+    - .fdo.container-build@ubuntu
+  stage: container
+  variables:
+    GIT_STRATEGY: none # no need to pull the whole tree for rebuilding the image
+
 container_fedora:
   extends:
     - .fedora
@@ -109,6 +148,14 @@ container_coverity:
   variables:
     GIT_STRATEGY: none
 
+build_on_ubuntu:
+  extends:
+    - .ubuntu
+    - .not_coverity
+    - .fdo.distribution-image@ubuntu
+    - .build
+  stage: build
+
 .build_on_fedora:
   extends:
     - .fedora