diff --git a/.github/actions/configure-cmake/action.yaml b/.github/actions/configure-cmake/action.yaml
index c368d6b8c..c1148acc9 100644
--- a/.github/actions/configure-cmake/action.yaml
+++ b/.github/actions/configure-cmake/action.yaml
@@ -46,6 +46,15 @@ runs:
       run: |
         # Source the container entrypoint script
         . /entrypoint.sh
+
+        # Load Spack-installed GCC if necessary
+        # FIXME: The entrypoint.sh script should include the adjustment of PATH
+        #        such that $(which g++) refers to the Spack-installed GCC.
+        if [ "$CPP_COMPILER" == "g++" ]; then
+          spack load gcc@15.2
+          CPP_COMPILER=$(which g++)
+        fi
+
         cd "$GITHUB_WORKSPACE/$BUILD_PATH"
         SOURCE_DIR="$GITHUB_WORKSPACE/$SOURCE_PATH"
         echo "Configuring with CMake preset: $PRESET"
diff --git a/.github/workflows/cmake-build.yaml b/.github/workflows/cmake-build.yaml
index dcba5940f..91cf79b34 100644
--- a/.github/workflows/cmake-build.yaml
+++ b/.github/workflows/cmake-build.yaml
@@ -217,7 +217,7 @@ jobs:
 
     - name: Configure CMake
       id: configure
-      uses: Framework-R-D/phlex/.github/actions/configure-cmake@main
+      uses: knoepfel/phlex/.github/actions/configure-cmake@cpp23-for-containers
       with:
         source-path: ${{ env.local_checkout_path }}
         build-path: ${{ env.local_build_path }}
diff --git a/.github/workflows/codeql-analysis.yaml b/.github/workflows/codeql-analysis.yaml
index cf40e3a0a..2e9fb68e3 100644
--- a/.github/workflows/codeql-analysis.yaml
+++ b/.github/workflows/codeql-analysis.yaml
@@ -98,7 +98,6 @@ jobs:
       - name: Verify compile_commands.json (C++ only)
         if: matrix.language == 'cpp'
         run: |
-          set -euo pipefail
           if [ ! -f "$CODEQL_EXTRACTOR_CPP_COMPILATION_DATABASE" ]; then
             echo "Expected compile_commands.json at $CODEQL_EXTRACTOR_CPP_COMPILATION_DATABASE" >&2
             exit 1
@@ -159,7 +158,6 @@ jobs:
       - name: Check CodeQL SARIF for new or resolved alerts
         id: check_codeql
         run: |
-          set -euo pipefail
           ARGS=(
             --sarif "$GITHUB_WORKSPACE/sarif"
             --min-level "${CODEQL_MIN_LEVEL}"
diff --git a/CMakePresets.json b/CMakePresets.json
index 79b4a20aa..dc1d73c27 100644
--- a/CMakePresets.json
+++ b/CMakePresets.json
@@ -5,7 +5,7 @@
             "hidden": false,
             "cacheVariables": {
                 "CMAKE_EXPORT_COMPILE_COMMANDS": "YES",
-                "CMAKE_CXX_STANDARD": "20",
+                "CMAKE_CXX_STANDARD": "23",
                 "CMAKE_CXX_STANDARD_REQUIRED": "YES",
                 "CMAKE_CXX_EXTENSIONS": "NO",
                 "CMAKE_PROJECT_TOP_LEVEL_INCLUDES": "CetProvideDependency"
diff --git a/ci/Dockerfile b/ci/Dockerfile
index e8a409e76..9b027cab7 100644
--- a/ci/Dockerfile
+++ b/ci/Dockerfile
@@ -15,7 +15,7 @@
 
 ARG parallelism=18
 
-FROM gcc:15.2.0 AS base
+FROM ubuntu:24.04 AS base
 ARG parallelism
 
 ENV DEBIAN_FRONTEND=noninteractive
@@ -27,6 +27,33 @@ ENV PHLEX_SPACK_ENV=/opt/spack-environments/phlex-ci
 
 SHELL ["/bin/bash", "-c"]
 
+########################################################################
+# Install essential build tools, git, and Python
+
+RUN <<'INSTALL_ESSENTIALS'
+set -euo pipefail
+
+apt-get update
+apt-get install -y --no-install-recommends \
+  build-essential \
+  ca-certificates \
+  curl \
+  file \
+  git \
+  gnupg2 \
+  libcurl4-openssl-dev \
+  locales-all \
+  python3 \
+  python3-dev \
+  python3-pip \
+  unzip
+apt-get clean
+rm -rf /var/lib/apt/lists/*
+
+# Make python3 available as 'python'
+update-alternatives --install /usr/bin/python python /usr/bin/python3 1
+INSTALL_ESSENTIALS
+
 ########################################################################
 # Install locale support required by tooling and Spack
 
@@ -117,13 +144,31 @@ set -euo pipefail
 # Register binary caches for Spack installs and optional local cache volume
 . /spack/share/spack/setup-env.sh
 
-spack mirror add --type binary phlex-ci-scisoft https://scisoft.fnal.gov/scisoft/phlex-dev-build-cache
+spack mirror add --type binary phlex-ci-scisoft https://scisoft.fnal.gov/scisoft/spack-packages/phlex-dev
 
 if [ -d "/build-cache" ]; then
   spack --timestamp mirror add --type binary phlex-ci-local /build-cache
 fi
 CONFIGURE_SPACK_MIRRORS
 
+########################################################################
+# Install GCC 15.2
+
+RUN <<'INSTALL_GCC'
+set -euo pipefail
+
+# Install GCC in its own layer to improve cache reuse
+. /spack/share/spack/setup-env.sh
+
+spack --timestamp install --fail-fast -j $parallelism -p 1 \
+  --no-check-signature gcc@15.2 target=x86_64_v3
+
+# Make GCC external to simplify concretization
+spack compiler find $(spack location -i gcc@15.2)
+
+spack clean -dfs
+INSTALL_GCC
+
 ########################################################################
 # Create, activate, and concretize the phlex-ci Spack environment
 
diff --git a/ci/spack.yaml b/ci/spack.yaml
index 616d9fdfa..04e95090a 100644
--- a/ci/spack.yaml
+++ b/ci/spack.yaml
@@ -5,14 +5,13 @@ spack:
   - cmake
   - lcov
   - ninja
-  - python
   - py-gcovr
-  - py-numpy
   - py-pip # Needed temporarily for ruff installation
   - |
-      llvm@21.1.4: +zstd +llvm_dylib +link_llvm_dylib targets=x86
+      llvm@21.1.4: +zstd +llvm_dylib +link_llvm_dylib ~lldb targets=x86
 
   view: true
+
   concretizer:
     unify: true
 
@@ -29,7 +28,7 @@ spack:
     phlex:
       require:
       - "+form"
-      - "cxxstd=20"
+      - "cxxstd=23"
       - "%gcc@15"
 
 
@@ -49,4 +48,8 @@ spack:
     root:
       require:
       - "~x"  # No graphics libraries required
-      - "cxxstd=20"
+      # FIXME: davix requires openssl headers which are not currently
+      #        available in the container.  They are also not needed
+      #        for Phlex development.  This should be rectified.
+      - "~davix"
+      - "cxxstd=23"