Update .gitlab-ci.yml file

.gitlab-ci.yml

 buildtest:
-  image: debian:stretch
+  image: debian:bullseye
   stage: test
   script:
     - apt update
-    - apt install -y wget
-    - 'echo "deb http://www.deb-multimedia.org stretch main non-free" >> /etc/apt/sources.list'
-    - wget https://www.deb-multimedia.org/pool/main/d/deb-multimedia-keyring/deb-multimedia-keyring_2016.8.1_all.deb
-    - dpkg -i deb-multimedia-keyring_2016.8.1_all.deb
-    - apt update
-    - uname -a
     - apt install -y locales-all git build-essential  make libavformat-dev libavcodec-dev libavfilter-dev libavutil-dev libxml2-dev libcurl4-openssl-dev
     - make
   artifacts: