github-actions: Add focal backend job.

No new sets of change in this commit that are explained two commits
before this one, which add bionic-backend-frontend.

.github/workflows/zulip-ci.yml

@@ -154,3 +154,87 @@ jobs:
     # config, however, it might be the case that GitHub Notifications
     # make this unnesscary. More details on settings to configure it:
     # https://help.github.com/en/github/managing-subscriptions-and-notifications-on-github/configuring-notifications#github-actions-notification-options
+
+  focal_backend:
+    name: Ubuntu 20.04 Focal (Python 3.8, backend)
+    runs-on: ubuntu-latest
+
+    # This docker image was created by a generated Dockerfile at:
+    #   tools/ci/images/focal/Dockerfile
+    # Focal ships with Python 3.8.2.
+    container: mepriyank/actions:focal
+    env:
+      # GitHub Actions sets HOME to /github/home which causes
+      # problem later in provison and frontend test that runs
+      # tools/setup/postgrest-init-dev-db because of .pgpass.
+      # psql expects .pgpass to be at /home/github/.pgpass and
+      # setting home to `/home/github/` ensures it written there.
+      HOME: /home/github/
+
+    steps:
+      - name: Add required permissions
+        run: |
+          # The checkout actions doesn't clone to ~/zulip or allow
+          # us to use the path option to clone outside the current
+          # /__w/zulip/zulip directory. Since this directory is owned
+          # by root we need to change it's ownership to allow the
+          # github user to clone the code here.
+          # Note: /__w/ is a docker volume mounted to $GITHUB_WORKSPACE
+          # which is /home/runner/work/.
+          sudo chown -R github .
+
+          # This is the GitHub Actions specific cache directory the
+          # the current github user must be able to access for the
+          # cache action to work. It is owned by root currently.
+          sudo chmod -R 0777 /__w/_temp/
+
+      - uses: actions/checkout@v2
+
+      - name: Create cache directories
+        run: |
+          dirs=(/srv/zulip-{npm,venv,emoji}-cache)
+          sudo mkdir -p "${dirs[@]}"
+          sudo chown -R github "${dirs[@]}"
+
+      - name: Restore node_modules cache
+        uses: actions/cache@v2
+        with:
+          path: /srv/zulip-npm-cache
+          key: v1-yarn-deps-${{ github.job }}-${{ hashFiles('package.json') }}-${{ hashFiles('yarn.lock') }}
+          restore-keys: v1-yarn-deps-${{ github.job }}
+
+      - name: Restore python cache
+        uses: actions/cache@v2
+        with:
+          path: /srv/zulip-venv-cache
+          key: v1-venv-${{ github.job }}-${{ hashFiles('requirements/thumbor-dev.txt') }}-${{ hashFiles('requirements/dev.txt') }}
+          restore-keys: v1-venv-${{ github.job }}
+
+      - name: Restore emoji cache
+        uses: actions/cache@v2
+        with:
+          path: /srv/zulip-emoji-cache
+          key: v1-emoji-${{ github.job }}-${{ hashFiles('tools/setup/emoji/emoji_map.json') }}-${{ hashFiles('tools/setup/emoji/build_emoji') }}-${{ hashFiles('tools/setup/emoji/emoji_setup_utils.py') }}-${{ hashFiles('tools/setup/emoji/emoji_names.py') }}-${{ hashFiles('package.json') }}
+          restore-keys: v1-emoji-${{ github.job }}
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+
+          # Install moreutils so we can use `ts` and `mispipe` in the following.
+          sudo apt-get install -y moreutils
+
+          # This is the main setup job for the test suite
+          mispipe "tools/ci/setup-backend --skip-dev-db-build" ts
+
+          # Cleaning caches is mostly unnecessary in GitHub Actions,
+          # because most builds don't get to write to the cache.
+          # mispipe "scripts/lib/clean-unused-caches --verbose --threshold 0 2>&1" ts
+
+      - name: Run backend tests
+        run: |
+          . /srv/zulip-py3-venv/bin/activate && \
+          mispipe "./tools/ci/backend 2>&1" ts
+
+      - name: Check development database build
+        run: mispipe "tools/ci/setup-backend" ts