diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 4f61ab2d9b..fdf5f48dc2 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -61,6 +61,7 @@ jobs:
           # shellcheck disable=SC2086
           ./firedrake/scripts/firedrake-install \
             $COMPLEX \
+            --petsc-int-type=int64 \
             --honour-petsc-dir \
             --mpicc="$MPICH_DIR"/mpicc \
             --mpicxx="$MPICH_DIR"/mpicxx \
@@ -150,3 +151,12 @@ jobs:
       tag: ${{ needs.docker_tag.outputs.tag }}
       status: ${{ needs.build.result }}
     secrets: inherit
+  docker_testing:
+    # Strictly for testing!
+    uses: ./.github/workflows/docker_reuse.yml
+    with:
+      target: firedrake-test
+      tag: latest
+      dockerfile: docker/Dockerfile.testing
+    secrets: inherit
+
diff --git a/docker/Dockerfile.testing b/docker/Dockerfile.testing
new file mode 100644
index 0000000000..6a13b1f417
--- /dev/null
+++ b/docker/Dockerfile.testing
@@ -0,0 +1,168 @@
+# DockerFile for an environment into which firedrake can be installed.
+
+FROM ubuntu:24.04
+
+# Update and install required packages for Firedrake
+USER root
+SHELL ["/bin/bash", "-o", "pipefail", "-c"]
+RUN apt-get update \
+    && apt-get -y dist-upgrade \
+    && DEBIAN_FRONTEND=noninteractive apt-get -y install tzdata \
+    && apt-get -y install curl vim docker.io \
+                 openssh-client build-essential autoconf automake \
+                 cmake gfortran git libopenblas-serial-dev \
+                 libtool python3-dev python3-pip python3-tk python3-venv \
+                 python3-requests zlib1g-dev libboost-dev sudo gmsh \
+                 bison flex ninja-build \
+                 libocct-ocaf-dev libocct-data-exchange-dev \
+                 swig graphviz \
+                 libcurl4-openssl-dev libxml2-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# Use a more sane locale
+ENV LC_ALL C.UTF-8
+
+# Change the `ubuntu` user to `firedrake`
+# and ensure that we do not run as root on self-hosted systems
+RUN usermod -d /home/firedrake -m ubuntu && \
+    usermod -l firedrake ubuntu && \
+    groupmod -n firedrake ubuntu && \
+    usermod -aG sudo firedrake && \
+    echo "firedrake:docker" | chpasswd && \
+    echo "firedrake ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers && \
+    ldconfig
+
+USER firedrake
+WORKDIR /home/firedrake
+
+# Fetch PETSc, SLEPc and eigen
+RUN git clone https://github.com/firedrakeproject/petsc.git
+RUN git clone https://github.com/firedrakeproject/slepc.git
+
+# Build MPICH manually because we don't want PETSc to build it twice
+RUN bash -c 'cd petsc; \
+    ./configure \
+        --with-64-bit-indices=1 \
+        --COPTFLAGS=-O3 -march=native -mtune=native \
+        --CXXOPTFLAGS=-O3 -march=native -mtune=native \
+        --FOPTFLAGS=-O3 -march=native -mtune=native \
+        --with-c2html=0 \
+        --with-debugging=0 \
+        --with-fortran-bindings=0 \
+        --with-make-np=12 \
+        --with-shared-libraries=1 \
+        --with-zlib \
+        --download-fftw \
+        --download-hdf5 \
+        --download-hwloc \
+        --download-hypre \
+        --download-metis \
+        --download-mumps \
+        --download-mpich \
+        --download-mpich-device=ch3:sock \
+        --download-netcdf \
+        --download-pastix \
+        --download-pnetcdf \
+        --download-ptscotch \
+        --download-scalapack \
+        --download-suitesparse \
+        --download-superlu_dist \
+        PETSC_ARCH=packages; \
+        mv packages/include/petscconf.h packages/include/old_petscconf.nope;'
+# Don't run make here, we only want MPICH and HWLOC
+# It is also necessary to move `petscconf.h` so packages isn't treated like a working PETSc
+
+# Build default Firedrake PETSc
+RUN bash -c 'export PACKAGES=/home/firedrake/petsc/packages; \
+    cd petsc; \
+    ./configure \
+        --with-64-bit-indices=1 \
+        --COPTFLAGS=-O3 -march=native -mtune=native \
+        --CXXOPTFLAGS=-O3 -march=native -mtune=native \
+        --FOPTFLAGS=-O3 -march=native -mtune=native \
+        --with-c2html=0 \
+        --with-debugging=0 \
+        --with-fortran-bindings=0 \
+        --with-make-np=12 \
+        --with-shared-libraries=1 \
+        --with-bison \
+        --with-flex \
+        --with-zlib \
+        --with-fftw-dir=$PACKAGES \
+        --with-hdf5-dir=$PACKAGES \
+        --with-hwloc-dir=$PACKAGES \
+        --with-hypre-dir=$PACKAGES \
+        --with-metis-dir=$PACKAGES \
+        --with-mpi-dir=$PACKAGES \
+        --with-mumps-dir=$PACKAGES \
+        --with-netcdf-dir=$PACKAGES \
+        --with-pastix-dir=$PACKAGES \
+        --with-pnetcdf-dir=$PACKAGES \
+        --with-ptscotch-dir=$PACKAGES \
+        --with-scalapack-dir=$PACKAGES \
+        --with-suitesparse-dir=$PACKAGES \
+        --with-superlu_dist-dir=$PACKAGES \
+        PETSC_ARCH=default; \
+    make PETSC_DIR=/home/firedrake/petsc PETSC_ARCH=default all;'
+
+# Build default Firedrake SLEPc
+RUN bash -c 'export PETSC_DIR=/home/firedrake/petsc; \
+    export PETSC_ARCH=default; \
+    cd slepc; \
+    ./configure; \
+    make SLEPC_DIR=/home/firedrake/slepc PETSC_DIR=/home/firedrake/petsc PETSC_ARCH=default;'
+
+# Additionally build complex PETSc for Firedrake
+RUN bash -c 'export PACKAGES=/home/firedrake/petsc/packages; \
+    cd petsc; \
+    ./configure \
+        --with-64-bit-indices=1 \
+        --COPTFLAGS=-O3 -march=native -mtune=native \
+        --CXXOPTFLAGS=-O3 -march=native -mtune=native \
+        --FOPTFLAGS=-O3 -march=native -mtune=native \
+        --with-c2html=0 \
+        --with-debugging=0 \
+        --with-fortran-bindings=0 \
+        --with-make-np=12 \
+        --with-scalar-type=complex \
+        --with-shared-libraries=1 \
+        --with-bison \
+        --with-flex \
+        --with-zlib \
+        --with-fftw-dir=$PACKAGES \
+        --with-hdf5-dir=$PACKAGES \
+        --with-hwloc-dir=$PACKAGES \
+        --with-metis-dir=$PACKAGES \
+        --with-mpi-dir=$PACKAGES \
+        --with-mumps-dir=$PACKAGES \
+        --with-netcdf-dir=$PACKAGES \
+        --with-pastix-dir=$PACKAGES \
+        --with-pnetcdf-dir=$PACKAGES \
+        --with-ptscotch-dir=$PACKAGES \
+        --with-scalapack-dir=$PACKAGES \
+        --with-suitesparse-dir=$PACKAGES \
+        --with-superlu_dist-dir=$PACKAGES \
+        PETSC_ARCH=complex; \
+    make PETSC_DIR=/home/firedrake/petsc PETSC_ARCH=complex all;'
+
+# Build complex Firedrake SLEPc
+RUN bash -c 'export PETSC_DIR=/home/firedrake/petsc; \
+    export PETSC_ARCH=complex; \
+    cd slepc; \
+    ./configure; \
+    make SLEPC_DIR=/home/firedrake/slepc PETSC_DIR=/home/firedrake/petsc PETSC_ARCH=complex;'
+
+# Clean up unnecessary files
+RUN rm -rf /home/firedrake/petsc/**/externalpackages \
+    && rm -rf /home/firedrake/petsc/src/docs \
+    && rm -f /home/firedrake/petsc/src/**/tutorials/output/* \
+    && rm -f /home/firedrake/petsc/src/**/tests/output/*
+
+# Set some useful environment variables
+ENV PETSC_DIR /home/firedrake/petsc
+ENV SLEPC_DIR /home/firedrake/slepc
+ENV MPICH_DIR /home/firedrake/petsc/packages/bin
+ENV HDF5_DIR /home/firedrake/petsc/packages
+ENV HDF5_MPI ON
+ENV OMP_NUM_THREADS 1
+ENV OPENBLAS_NUM_THREADS 1