feat: install most dependencies in cacheable layer, before adding main

Instead of requiring the whole stack to be reinstalled whenever a specific version (e.g. main) causes a cache fail, this aims to have stable default version with a second-stage installation of the potentially-modified versions of eicrecon and juggler. This should allow all builds to cache the first installation in virtually all cases, leading to faster builds.

containers/jug/dev.Dockerfile

@@ -134,6 +134,7 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
 set -e
 export CCACHE_DIR=/ccache
 source ${SPACK_ROOT}/share/spack/setup-env.sh
+mkdir -p /var/cache/spack/blobs/sha256/
 find /var/cache/spack/blobs/sha256/ -atime +7 -delete
 spack buildcache update-index eics3rw
 spack env activate --dir ${SPACK_ENV}
@@ -165,6 +166,7 @@ if [ "${EICRECON_VERSION}" != "28108da4a1e8919a05dfdb5f11e114800a2cbe96" ] ; the
   export EICRECON_VERSION=$(jq -r .sha /tmp/eicrecon.json)
   spack config add "packages:eicrecon::require:['@git.${EICRECON_VERSION}']"
 fi
+cat ${SPACK_ENV}/spack.yaml
 spack concretize --fresh --force --quiet
 make --jobs ${jobs} --keep-going --directory /opt/spack-environment \
   SPACK_ENV=${SPACK_ENV} \