workflows: Run unit tests in our tasks container

This reduces our tools like `ruff` to a single source of truth (as all
our other projects already run their unit tests and linting in the tasks
container). It also removes a lot of moving parts only relevant for CI.
In practice, us developers run the unit tests in toolbox or our own dev
machines anyway.

Move building the guide in the release workflow to the tasks container
as well.

.github/workflows/release.yml

@@ -52,7 +52,7 @@ jobs:
     permissions: {}
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/cockpit-project/unit-tests
+      image: quay.io/cockpit/tasks:latest
       options: --user root
     steps:
       - name: Checkout website repository

.github/workflows/unit-tests.yml

@@ -4,39 +4,34 @@ jobs:
   unit-tests:
     runs-on: ubuntu-22.04
     permissions: {}
+    container:
+      image: quay.io/cockpit/tasks:latest
+      options: --user 1001
     strategy:
       matrix:
-        startarg:
-          - { make: 'check-memory', cc: 'gcc', tag: 'latest' }
-          - { make: 'distcheck', cc: 'clang', tag: 'latest' }
+        scenario:
+          - { make: 'check-memory', cc: 'gcc' }
+          - { make: 'distcheck', cc: 'clang' }
           # this runs static code checks, unlike distcheck
-          - { make: 'check', cc: 'gcc', tag: 'latest' }
-          - { make: 'pytest-cov', cc: 'gcc', tag: 'latest' }
+          - { make: 'check', cc: 'gcc' }
+          - { make: 'pytest-cov', cc: 'gcc' }
       fail-fast: false
     timeout-minutes: 60
+    env:
+      FORCE_COLOR: 1
+      TEST_BROWSER: firefox
+      CFLAGS: '-O2 -gdwarf-4'
     steps:
       - name: Clone repository
         uses: actions/checkout@v4
         with:
           # need this to also fetch tags
           fetch-depth: 0
+          submodules: true
 
-      - name: Build unit test container if it changed
-        run: |
-          changes=$(git diff --name-only origin/${{ github.event.pull_request.base.ref }}..HEAD -- containers/unit-tests/)
-          if [ -n "${changes}" ]; then
-            containers/unit-tests/build
-          fi
-
-      - name: Run unit-tests container
+      - name: Run unit test
         timeout-minutes: 30
         # HACK: -gdwarf-4 is for clang: https://bugs.kde.org/show_bug.cgi?id=452758
         run: |
-          containers/unit-tests/start \
-              --verbose \
-              --env=FORCE_COLOR=1 \
-              --env=CC='${{ matrix.startarg.cc }}' \
-              --env=CFLAGS='-O2 -gdwarf-4' \
-              --env=EXTRA_DISTCHECK_CONFIGURE_FLAGS='${{ matrix.startarg.distcheck_flags }}' \
-              --image-tag='${{ matrix.startarg.tag }}' \
-              --make '${{ matrix.startarg.make }}'
+            ./autogen.sh
+            make -j$(nproc) CC='${{ matrix.scenario.cc }}' '${{ matrix.scenario.make }}'