diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3a188bf..04e03fb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,6 +20,8 @@ concurrency:
 
 env:
   GIT_FETCH_JOBS: 8
+  NET_RETRY_COUNT: 5
+  DEFAULT_BUILD_VARIANT: debug,release
 
 jobs:
   posix:
@@ -31,150 +33,202 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          # Linux, gcc
           - toolset: gcc-4.4
             cxxstd: "98,0x"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: g++-4.4
+            install:
+              - g++-4.4
+            sources:
+              - "ppa:ubuntu-toolchain-r/test"
           - toolset: gcc-4.6
             cxxstd: "03,0x"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: g++-4.6
+            install:
+              - g++-4.6
+            sources:
+              - "ppa:ubuntu-toolchain-r/test"
           - toolset: gcc-4.7
             cxxstd: "03,11"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: g++-4.7
+            install:
+              - g++-4.7
           - toolset: gcc-4.8
             cxxstd: "03,11"
             os: ubuntu-18.04
-            install: g++-4.8
+            install:
+              - g++-4.8
           - toolset: gcc-4.9
             cxxstd: "03,11"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: g++-4.9
+            install:
+              - g++-4.9
           - toolset: gcc-5
             cxxstd: "03,11,14,1z"
             os: ubuntu-20.04
             container: ubuntu:16.04
+            install:
+              - g++-5
           - toolset: gcc-6
             cxxstd: "03,11,14,1z"
             os: ubuntu-18.04
-            install: g++-6
+            install:
+              - g++-6
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
             os: ubuntu-18.04
+            install:
+              - g++-7
           - toolset: gcc-8
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-18.04
-            install: g++-8
+            install:
+              - g++-8
           - toolset: gcc-9
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-18.04
+            install:
+              - g++-9
           - toolset: gcc-10
             cxxstd: "03,11,14,17,20"
             os: ubuntu-20.04
+            install:
+              - g++-10
           - toolset: gcc-11
             cxxstd: "03,11,14,17,20"
             os: ubuntu-20.04
-            install: g++-11
+            install:
+              - g++-11
+            sources:
+              - "ppa:ubuntu-toolchain-r/test"
           - name: UBSAN
             toolset: gcc-11
             cxxstd: "03,11,14,17,20"
             ubsan: 1
             os: ubuntu-20.04
-            install: g++-11
+            install:
+              - g++-11
+            sources:
+              - "ppa:ubuntu-toolchain-r/test"
 
+          # Linux, clang
           - toolset: clang
             compiler: clang++-3.5
             cxxstd: "03,11"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: clang-3.5
+            install:
+              - clang-3.5
           - toolset: clang
             compiler: clang++-3.6
             cxxstd: "03,11,14"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: clang-3.6
+            install:
+              - clang-3.6
           - toolset: clang
             compiler: clang++-3.7
             cxxstd: "03,11,14"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: clang-3.7
+            install:
+              - clang-3.7
           - toolset: clang
             compiler: clang++-3.8
             cxxstd: "03,11,14"
             os: ubuntu-20.04
             container: ubuntu:16.04
-            install: clang-3.8
+            install:
+              - clang-3.8
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "03,11,14"
             os: ubuntu-18.04
-            install: clang-3.9
+            install:
+              - clang-3.9
           - toolset: clang
             compiler: clang++-4.0
             cxxstd: "03,11,14"
             os: ubuntu-18.04
-            install: clang-4.0
+            install:
+              - clang-4.0
           - toolset: clang
             compiler: clang++-5.0
             cxxstd: "03,11,14,1z"
             os: ubuntu-18.04
-            install: clang-5.0
+            install:
+              - clang-5.0
           - toolset: clang
             compiler: clang++-6.0
             cxxstd: "03,11,14,17"
             os: ubuntu-18.04
-            install: clang-6.0
+            install:
+              - clang-6.0
           - toolset: clang
             compiler: clang++-7
             cxxstd: "03,11,14,17"
             os: ubuntu-18.04
-            install: clang-7
+            install:
+              - clang-7
+          # Note: clang-8 does not fully support C++20, so it is not compatible with libstdc++-8 in this mode
           - toolset: clang
             compiler: clang++-8
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-18.04
-            install: clang-8
+            install:
+              - clang-8
+              - g++-7
             gcc_toolchain: 7
           - toolset: clang
             compiler: clang++-9
             cxxstd: "03,11,14,17,2a"
             os: ubuntu-20.04
-            install: clang-9
+            install:
+              - clang-9
           - toolset: clang
             compiler: clang++-10
             cxxstd: "03,11,14,17,20"
             os: ubuntu-20.04
+            install:
+              - clang-10
           - toolset: clang
             compiler: clang++-11
             cxxstd: "03,11,14,17,20"
             os: ubuntu-20.04
+            install:
+              - clang-11
           - toolset: clang
             compiler: clang++-12
             cxxstd: "03,11,14,17,20"
             os: ubuntu-20.04
+            install:
+              - clang-12
           - toolset: clang
             compiler: clang++-12
             cxxstd: "03,11,14,17,20"
-            os: ubuntu-20.04
-            install: libc++-12-dev libc++abi-12-dev
             cxxflags: -stdlib=libc++
             linkflags: -stdlib=libc++
+            os: ubuntu-20.04
+            install:
+              - clang-12
+              - libc++-12-dev
+              - libc++abi-12-dev
           - name: UBSAN
             toolset: clang
             compiler: clang++-12
             cxxstd: "03,11,14,17,20"
-            ubsan: 1
-            os: ubuntu-20.04
-            install: libc++-12-dev libc++abi-12-dev
             cxxflags: -stdlib=libc++
             linkflags: -stdlib=libc++
+            ubsan: 1
+            os: ubuntu-20.04
+            install:
+              - clang-12
+              - libc++-12-dev
+              - libc++abi-12-dev
 
           - toolset: clang
             cxxstd: "03,11,14,17,2a"
@@ -188,20 +242,77 @@ jobs:
     container: ${{matrix.container}}
 
     steps:
-      - name: Setup container
-        if: matrix.container
+      - name: Setup environment
         run: |
-            echo "GHA_CONTAINER=${{matrix.container}}" >> $GITHUB_ENV
-            apt-get -o Acquire::Retries=3 update && DEBIAN_FRONTEND=noninteractive apt-get -o Acquire::Retries=3 install -y sudo software-properties-common tzdata wget curl apt-transport-https make apt-file unzip libssl-dev build-essential autotools-dev autoconf automake g++ libc++-helpers python ruby cpio gcc-multilib g++-multilib pkgconf python3 ccache libpython-dev git cmake
+            if [ -f "/etc/debian_version" ]
+            then
+                echo "DEBIAN_FRONTEND=noninteractive" >> $GITHUB_ENV
+                export DEBIAN_FRONTEND=noninteractive
+            fi
+            if [ -n "${{matrix.container}}" ]
+            then
+                echo "GHA_CONTAINER=${{matrix.container}}" >> $GITHUB_ENV
+                if [ -f "/etc/debian_version" ]
+                then
+                    apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
+                    apt-get -o Acquire::Retries=$NET_RETRY_COUNT install -y sudo software-properties-common tzdata wget curl apt-transport-https ca-certificates make build-essential g++ python python3 git cmake
+                fi
+            fi
+            git config --global pack.threads 0
 
       - uses: actions/checkout@v2
 
       - name: Install packages
         if: matrix.install
         run: |
-            for i in {1..3}; do sudo -E apt-add-repository -y "ppa:ubuntu-toolchain-r/test" && break || sleep 2; done
-            sudo apt-get update
-            sudo apt-get install -y ${{matrix.install}}
+            SOURCE_KEYS=(${{join(matrix.source_keys, ' ')}})
+            SOURCES=(${{join(matrix.sources, ' ')}})
+            for key in "${SOURCE_KEYS[@]}"
+            do
+                for i in {1..$NET_RETRY_COUNT}
+                do
+                    wget -O - "$key" | sudo apt-key add - && break || sleep 2
+                done
+            done
+            if [ ${#SOURCES[@]} -gt 0 ]
+            then
+                APT_ADD_REPO_COMMON_ARGS=("-y")
+                APT_ADD_REPO_HAS_SOURCE_ARGS=0
+                SOFTWARE_PROPERTIES_VERSION="$(dpkg-query --showformat='${Version}' --show software-properties-common)"
+                if dpkg --compare-versions "$SOFTWARE_PROPERTIES_VERSION" ge "0.96.24.20"
+                then
+                    APT_ADD_REPO_COMMON_ARGS+=("-n")
+                fi
+                if dpkg --compare-versions "$SOFTWARE_PROPERTIES_VERSION" ge "0.98.10"
+                then
+                    APT_ADD_REPO_HAS_SOURCE_ARGS=1
+                fi
+                for source in "${SOURCES[@]}"
+                do
+                    for i in {1..$NET_RETRY_COUNT}
+                    do
+                        APT_ADD_REPO_ARGS=("${APT_ADD_REPO_COMMON_ARGS[@]}")
+                        if [ $APT_ADD_REPO_HAS_SOURCE_ARGS -ne 0 ]
+                        then
+                            case "$source" in
+                            "ppa:"*)
+                                APT_ADD_REPO_ARGS+=("-P")
+                                ;;
+                            "deb "*)
+                                APT_ADD_REPO_ARGS+=("-S")
+                                ;;
+                            *)
+                                APT_ADD_REPO_ARGS+=("-U")
+                                ;;
+                            esac
+                        fi
+                        APT_ADD_REPO_ARGS+=("$source")
+                        sudo -E apt-add-repository "${APT_ADD_REPO_ARGS[@]}" && break || sleep 2
+                    done
+                done
+            fi
+            sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
+            sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT install -y ${{join(matrix.install, ' ')}}
 
       - name: Setup GCC Toolchain
         if: matrix.gcc_toolchain
@@ -249,24 +360,32 @@ jobs:
             fi
             DEPINST_ARGS+=("$LIBRARY")
             python tools/boostdep/depinst/depinst.py "${DEPINST_ARGS[@]}"
-            ./bootstrap.sh
-            ./b2 headers
-
-      - name: Create user-config.jam
-        if: matrix.cmake_tests == '' && matrix.compiler != ''
-        run: |
-            echo -n "using ${{matrix.toolset}} : : ${{matrix.compiler}}" > ~/user-config.jam
-            if [ -n "$GCC_TOOLCHAIN_ROOT" ]
+            if [ -z "${{matrix.cmake_tests}}" ]
             then
-                echo -n " : <compileflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\" <linkflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\"" >> ~/user-config.jam
+                ./bootstrap.sh
+                ./b2 headers
+                if [ -n "${{matrix.compiler}}" -o -n "$GCC_TOOLCHAIN_ROOT" ]
+                then
+                    echo -n "using ${{matrix.toolset}} : : ${{matrix.compiler}}" > ~/user-config.jam
+                    if [ -n "$GCC_TOOLCHAIN_ROOT" ]
+                    then
+                        echo -n " : <compileflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\" <linkflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\"" >> ~/user-config.jam
+                    fi
+                    echo " ;" >> ~/user-config.jam
+                fi
             fi
-            echo " ;" >> ~/user-config.jam
 
       - name: Run tests
         if: matrix.cmake_tests == ''
         run: |
             cd ../boost-root
-            B2_ARGS=("-j" "$BUILD_JOBS" "toolset=${{matrix.toolset}}" "cxxstd=${{matrix.cxxstd}}" "variant=debug,release")
+            B2_ARGS=("-j" "$BUILD_JOBS" "toolset=${{matrix.toolset}}" "cxxstd=${{matrix.cxxstd}}")
+            if [ -n "${{matrix.build_variant}}" ]
+            then
+                B2_ARGS+=("variant=${{matrix.build_variant}}")
+            else
+                B2_ARGS+=("variant=$DEFAULT_BUILD_VARIANT")
+            fi
             if [ -n "${{matrix.threading}}" ]
             then
                 B2_ARGS+=("threading=${{matrix.threading}}")