Switch ubuntu:latest to ubuntu:rolling for Gitlab CI

Ubuntu:rolling is the latest Ubuntu release (LTS and/or non-LTS).
Since debian:latest already provides a reasonable base for testing
firejail builds on older Debian-based systems, use ubuntu:rolling
to test on relatively new systems.

.gitlab-ci.yml

@@ -1,11 +1,11 @@
 # Basic notes: builds firejail on 5 different systems for 2 package systems:
 # 1. Debian-based systems. Use debian:jessie to ensure reasonable backwards
-# compat and ubuntu:latest for new setups
+# compat and ubuntu:rolling for new setups
 # 2. Redhat-based systems. Use centos:latest for reasonable backwards compat
 # and fedora:latest for new setups
 # 3. Alpine for installing directly from source
 build_ubuntu_package:
-    image: ubuntu:latest
+    image: ubuntu:rolling
     script:
     - apt-get update -qq
     - apt-get install -y -qq build-essential lintian