Add a Dockerfile to build the tutorial buildcache locally

container/buildcache.dockerfile

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+# Multi-stage variant of ./Dockerfile.
+#
+# Instead of pulling a prebuilt buildcache, this builds the buildcache
+# locally from container/spack.yaml in a "builder" stage, then copies
+# it into the tutorial image. Use it to try out new specs: edit
+# container/spack.yaml and rebuild.
+#
+#   docker build -f container/buildcache.dockerfile -t tutorial:experimental container
+#
+
+# ---- stage 1: build the buildcache ----
+FROM ghcr.io/spack/tutorial-ubuntu-26.04:v2026-06-09 AS builder
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV SPACK_ROOT=/spack/spack
+ENV PATH=$SPACK_ROOT/bin:$PATH
+ENV SPACK_COMMIT=63943396d634faa92cb59474df3558ec9b9b2425
+ENV SPACK_PACKAGES_COMMIT=688a7045fbcaea7b4965ac51e266408a0b591b9d
+
+# Base image has no spack, so clone it and pin to a specific commit.
+RUN git clone --depth 1 https://github.com/spack/spack.git $SPACK_ROOT && \
+    git -C $SPACK_ROOT fetch --depth 1 origin $SPACK_COMMIT && \
+    git -C $SPACK_ROOT checkout $SPACK_COMMIT
+
+# Pin the builtin package repository to a specific commit.
+RUN spack repo update builtin --commit $SPACK_PACKAGES_COMMIT
+
+COPY spack.yaml /opt/spack-environment/spack.yaml
+
+RUN --mount=type=cache,target=/home/software/spack \
+    spack -e /opt/spack-environment concretize -f && \
+    spack -e /opt/spack-environment install --fail-fast
+
+RUN --mount=type=cache,target=/home/software/spack \
+    spack -e /opt/spack-environment buildcache push --unsigned --update-index /mirror && \
+    chmod -R go+r /mirror
+
+# ---- stage 2: the tutorial image ----
+FROM ghcr.io/spack/tutorial-ubuntu-26.04:v2026-06-09
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Same tooling as ./Dockerfile, minus rclone (no longer needed).
+RUN apt-get update -y && \
+    apt-get install -y --no-install-recommends \
+    bash-completion \
+    ca-certificates \
+    emacs \
+    jq \
+    less \
+    vim
+
+# Bring in the buildcache produced by the builder stage.
+COPY --from=builder /mirror /mirror
+
+COPY packages.yaml /etc/spack/packages.yaml
+COPY config.yaml /etc/spack/config.yaml
+COPY concretizer.yaml /etc/spack/concretizer.yaml
+
+RUN useradd -ms /bin/bash spack && \
+    chmod -R go+r /etc/spack
+
+USER spack
+
+WORKDIR /home/spack
+
+ENTRYPOINT [ "bash" ]

container/spack.yaml

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+spack:
+  view: false
+
+  packages:
+    all:
+      require:
+      - target=x86_64_v3
+    tbb:
+      require: intel-tbb
+
+  specs:
+  - group: compiler
+    specs:
+    - gcc@16 build_type=Release +profiled +strip
+
+  - group: "gcc@15 specs"
+    specs:
+    - zlib-ng@2.2.4
+    - zlib-ng@2.0.7
+    - zlib-ng +ipo
+    - zlib-ng build_type=Debug
+    - zlib-ng %clang
+    - tcl ^zlib-ng@2.2.4
+    - tcl ^zlib-ng@2.0.7 %clang
+    - tcl ^zlib-ng %gcc@14
+    - hdf5+mpi^openmpi
+    - hdf5~mpi
+    - hdf5+hl+mpi ^mpich
+    - trilinos ^openmpi
+    - trilinos +hdf5 ^hdf5+hl+mpi ^mpich
+    - mpileaks
+    - lmod@8.7.67
+    - environment-modules
+    - macsio@1.1+scr ^scr@2.0.0~fortran ^silo~fortran ^hdf5~fortran
+    - julia ^llvm ~clang ~gold ~lldb ~lua ~polly compiler-rt=none libcxx=none libunwind=none targets=x86 ^openblas threads=openmp
+    - vim
+    override:
+      packages:
+        c:
+          prefer: [gcc@15]
+        cxx:
+          prefer: [gcc@15]
+        fortran:
+          prefer: [gcc@15]
+
+  - group: scalapacks
+    needs: [compiler]
+    specs:
+    - matrix:
+      - [netlib-scalapack]
+      - [^mpich, ^openmpi]
+      - [^openblas, ^netlib-lapack]
+    - matrix:
+      - [py-scipy ^openblas, armadillo ^openblas, netlib-lapack, openmpi, mpich, elpa ^mpich]
+    override:
+      packages:
+        c:
+          prefer: [gcc@16]
+        cxx:
+          prefer: [gcc@16]
+        fortran:
+          prefer: [gcc@16]
+
+  concretizer:
+    unify: false
+    reuse: false
+    targets:
+      granularity: generic
+
+  config:
+    installer: new
+    misc_cache: "$user_cache_path/cache"
+    db_lock_timeout: 120
+    shared_linking:
+      missing_library_policy: warn
+    install_tree:
+      root: /home/software/spack
+      padded_length: 256
+      projections:
+        all: "{architecture.platform}-{architecture.target}/{name}-{version}-{hash}"