Use debian again for the docker CI image

It has a new enough firefox now

packages/common/.gitlab-ci.yml

-image: registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v2
+image: registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v4
 
 before_script:
   - "sed -i 's|git@gitlab.tugraz.at:VPU|../..|g' .gitmodules"

packages/common/.gitlab-ci/Dockerfile

-FROM ubuntu:18.04
+FROM debian:buster
 
 ENV LANG C.UTF-8
 ENV DEBIAN_FRONTEND noninteractive
@@ -13,8 +13,8 @@ RUN curl -sL https://deb.nodesource.com/setup_12.x | bash - \
 
 RUN apt-get update && apt-get install -y \
     nodejs \
-    chromium-browser \
-    firefox \
+    chromium \
+    firefox-esr \
     composer \
     php-zip \
     rsync \

packages/common/.gitlab-ci/build.sh

 #!/bin/bash
 
 set -e
-TAG="registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v3"
+TAG="registry.gitlab.tugraz.at/vpu/webcomponents/common/main:v4"
 sudo docker build --tag "${TAG}" --file "Dockerfile" .
 echo "Now run: sudo docker push '$TAG'"