ci: Move upgrading git for bionic to docker image.

Just reduces the clutter in configuration file for GitHub Actions.

.github/workflows/zulip-ci.yml

@@ -40,18 +40,6 @@ jobs:
       HOME: /home/github/
 
     steps:
-      # Upgrade git before using checkout actions because it downloads
-      # the code using Rest API if git version is less than v2.18,
-      # which will cause errors in tests and provision. This is only
-      # required for bionic.
-      - name: Upgrade git for bionic
-        if: ${{ matrix.is_bionic }}
-        run: |
-          sudo apt-get install -y software-properties-common
-          sudo add-apt-repository ppa:git-core/ppa -y
-          sudo apt-get update
-          sudo apt-get install -y git
-
       - name: Add required permissions
         run: |
           # The checkout actions doesn't clone to ~/zulip or allow

tools/ci/Dockerfile.template

@@ -96,6 +96,18 @@ RUN apt-get update \
     libxml2-dev libxslt1-dev libpq-dev moreutils \
     {extra_packages}
 
+# Upgrade git if it is less than v2.18 because GitHub Actions'
+# checkout installs source code using Rest API as an optimization
+# if the version is less than v2.18, which causes failure in provision
+# and tests because of the lack of git being initialized.
+RUN export git_version=$(git --version | cut -d ' ' -f3 | cut -d 'v' -f2) && \
+    if dpkg --compare-versions $git_version lt 2.18; then \
+      sudo apt-get install -y software-properties-common && \
+      sudo add-apt-repository ppa:git-core/ppa -y && \
+      sudo apt-get update && \
+      sudo apt-get install -y git; \
+    fi
+
 ARG USERNAME=github
 RUN groupadd --gid 3434 $USERNAME \
   && useradd --uid 3434 --gid $USERNAME --shell /bin/bash --create-home $USERNAME \