Allow overriding Gitlab URL

Containerfile

@@ -45,6 +45,8 @@ ENTRYPOINT /lib/systemd/systemd
 # Gitlab-runner configuration options, may be freely overridden at
 # container image build time.
 ARG DEFAULT_JOB_IMAGE=registry.fedoraproject.org/fedora-minimal:latest
+# Allow image-builders to override the Gitlab URL
+ARG GITLAB_URL=https://gitlab.com/
 # Run nested containers in --privileged mode - required to allow building
 # container images using podman or buildah.  Otherwise may be set 'false'.
 ARG NESTED_PRIVILEGED=true
@@ -59,7 +61,7 @@ LABEL register="podman run -it --rm \
  --secret=REGISTRATION_TOKEN,type=env \
  -v ./config.toml:/etc/gitlab-runner/config.toml:Z \
  -e REGISTER_NON_INTERACTIVE=true \
- -e CI_SERVER_URL=https://gitlab.com/ \
+ -e CI_SERVER_URL=${GITLAB_URL}} \
  -e RUNNER_NAME=pipglr \
  -e RUNNER_EXECUTOR=docker \
  -e RUNNER_SHELL=bash \