ci: Build Docker images rather than OCI images if using podman

The dockerd instance on some (but not all) GitLab CI runners doesn’t
like the OCI output from the version of podman on Fedora 31, which is
causing a lot of spurious CI failures.

If whoever’s running `run-docker.sh` is using podman to emulate docker,
it needs to be told to output in Docker format, not OCI format.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

.gitlab-ci/run-docker.sh

@@ -88,8 +88,18 @@ fi
 TAG="registry.gitlab.gnome.org/gnome/glib/${base}:${base_version}"
 
 if [ $build == 1 ]; then
+        if docker --help |& grep -q podman; then
+                # Docker is actually implemented by podman, and its OCI output
+                # is incompatible with some of the dockerd instances on GitLab
+                # CI runners.
+                format="--format docker"
+        else
+                format=""
+        fi
+
         echo -e "\e[1;32mBUILDING\e[0m: ${base} as ${TAG}"
         sudo docker build \
+                ${format} \
                 --build-arg HOST_USER_ID="$UID" \
                 --tag "${TAG}" \
                 --file "${base}.Dockerfile" .