Run Linux CI on GCC 8 and clang 9

Those are the minimum supported versions. Only PCH builds are performed
to save build time.

build/jenkins/dockerfiles/bullseye-base.Dockerfile

@@ -0,0 +1,52 @@
+FROM debian:bullseye-slim
+
+RUN useradd -ms /bin/bash --uid 1006 builder
+
+# 0 A.D. dependencies.
+ARG DEBIAN_FRONTEND=noninteractive
+ARG DEBCONF_NOWARNINGS="yes"
+RUN apt-get -qqy update && apt-get install -qqy --no-install-recommends \
+      cmake \
+      curl \
+      libboost-dev \
+      libboost-filesystem-dev \
+      libcurl4-gnutls-dev \
+      libenet-dev \
+      libfmt-dev \
+      libfreetype6-dev \
+      libgloox-dev \
+      libgnutls28-dev \
+      libgtk-3-dev \
+      libicu-dev \
+      libminiupnpc-dev \
+      libogg-dev \
+      libopenal-dev \
+      libpng-dev \
+      libsdl2-dev \
+      libsodium-dev \
+      libvorbis-dev \
+      libwxgtk3.0-gtk3-dev \
+      libxml2-dev \
+      make \
+      m4 \
+      patch \
+      python3-dev \
+      python3-pip \
+      python-is-python3 \
+      subversion \
+      xz-utils \
+      zlib1g-dev \
+ && apt-get clean
+
+# Install git-lfs
+RUN curl -s https://packagecloud.io/install/repositories/github/git-lfs/script.deb.sh | bash
+RUN apt-get -qqy update && apt-get install -qqy --no-install-recommends git-lfs
+RUN git lfs install --system --skip-smudge
+
+# Install rust and Cargo via rustup
+USER builder
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --default-toolchain 1.51.0 -y
+ENV PATH="${PATH}:/home/builder/.cargo/bin"
+USER root
+
+ENV SHELL=/bin/bash

build/jenkins/dockerfiles/buster-base.Dockerfile

@@ -18,7 +18,6 @@ RUN apt-get -qqy update && apt-get install -qqy --no-install-recommends \
       libgnutls28-dev \
       libgtk-3-dev \
       libicu-dev \
-      libidn11-dev \
       libminiupnpc-dev \
       libogg-dev \
       libopenal-dev \
@@ -48,9 +47,9 @@ RUN git lfs install --system --skip-smudge
 
 # Install rust and Cargo via rustup
 USER builder
-RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --default-toolchain 1.51.0 -y
 ENV PATH="${PATH}:/home/builder/.cargo/bin"
 USER root
 
-ENV SHELL /bin/bash
+ENV SHELL=/bin/bash
 

build/jenkins/dockerfiles/clang8.Dockerfile

@@ -1,12 +0,0 @@
-FROM buster-base:latest
-
-ARG DEBIAN_FRONTEND=noninteractive
-ARG DEBCONF_NOWARNINGS="yes"
-RUN echo "deb https://deb.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/backports.list
-RUN apt-get update && apt-get install -qqy llvm-8 clang-8 lld-8 libclang-8-dev --no-install-recommends
-
-USER builder
-
-ENV CC clang-8
-ENV CXX clang++-8
-ENV LDFLAGS -fuse-ld=lld-8

build/jenkins/dockerfiles/clang9.Dockerfile

@@ -0,0 +1,11 @@
+FROM bullseye-base:latest
+
+ARG DEBIAN_FRONTEND=noninteractive
+ARG DEBCONF_NOWARNINGS="yes"
+RUN apt-get update && apt-get install -qqy llvm-9 clang-9 lld-9 libclang-9-dev --no-install-recommends
+
+USER builder
+
+ENV CC=clang-9
+ENV CXX=clang++-9
+ENV LDFLAGS="-fuse-ld=lld-9"

build/jenkins/dockerfiles/gcc7.Dockerfile

@@ -1,12 +0,0 @@
-FROM buster-base:latest
-
-ARG DEBIAN_FRONTEND=noninteractive
-ARG DEBCONF_NOWARNINGS="yes"
-RUN apt-get install -qqy gcc-7 g++-7 llvm-7 libclang-7-dev --no-install-recommends
-
-USER builder
-
-ENV LIBCC gcc-7
-ENV LIBCXX g++-7
-ENV CC gcc-7
-ENV CXX g++-7

build/jenkins/dockerfiles/gcc8.Dockerfile

@@ -0,0 +1,12 @@
+FROM buster-base:latest
+
+ARG DEBIAN_FRONTEND=noninteractive
+ARG DEBCONF_NOWARNINGS="yes"
+RUN apt-get install -qqy gcc-8 g++-8 --no-install-recommends
+
+USER builder
+
+ENV LIBCC=gcc-8
+ENV LIBCXX=g++-8
+ENV CC=gcc-8
+ENV CXX=g++-8

build/jenkins/pipelines/linux.Jenkinsfile

@@ -15,7 +15,7 @@
  * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-// This pipeline builds the game on Linux (with min and max supported versions of GCC and clang) and runs tests.
+// This pipeline builds the game on Linux (with minimum supported versions of GCC and clang) and runs tests.
 
 pipeline {
 	// Stop previous build in pull requests, but not in branches
@@ -37,6 +37,7 @@ pipeline {
 			steps {
 				discoverGitReferenceBuild()
 				sh "cd build/jenkins/dockerfiles/ && docker build -t buster-base -f buster-base.Dockerfile ."
+				sh "cd build/jenkins/dockerfiles/ && docker build -t bullseye-base -f bullseye-base.Dockerfile ."
 			}
 		}
 		stage("Linux Build") {
@@ -46,18 +47,14 @@ pipeline {
 				axes {
 					axis {
 						name 'JENKINS_COMPILER'
-						values 'gcc7' //, 'gcc14', 'clang8', 'clang18'
-					}
-					axis {
-						name 'JENKINS_PCH'
-						values 'pch' //, 'no-pch'
+						values 'gcc8', 'clang9'
 					}
 				}
 
 				agent {
 					dockerfile {
 						label 'LinuxAgent'
-						customWorkspace "workspace/${JENKINS_COMPILER}-${JENKINS_PCH}"
+						customWorkspace "workspace/${JENKINS_COMPILER}-pch"
 						dir 'build/jenkins/dockerfiles'
 						filename "${JENKINS_COMPILER}.Dockerfile"
 					}
@@ -71,13 +68,7 @@ pipeline {
 
 							sh "libraries/build-source-libs.sh 2> ${JENKINS_COMPILER}-prebuild-errors.log"
 
-							script {
-								if (env.JENKINS_PCH == "no-pch") {
-									sh "build/workspaces/update-workspaces.sh --jenkins-tests --without-pch 2>> ${JENKINS_COMPILER}-prebuild-errors.log"
-								} else {
-									sh "build/workspaces/update-workspaces.sh --jenkins-tests 2>> ${JENKINS_COMPILER}-prebuild-errors.log"
-								}
-							}
+							sh "build/workspaces/update-workspaces.sh --jenkins-tests 2>> ${JENKINS_COMPILER}-prebuild-errors.log"
 
 							script {
 								if (params.CLEANBUILD) {