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

@@ -154,6 +154,7 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
  && if [ -n ${EICRECON_VERSION} ] ; then                                \
       spack config add "packages:eicrecon::require:['@git.${EICRECON_VERSION}']" ; \
     fi                                                                  \
+ && cat ${SPACK_ENV}/spack.yaml \
  && spack concretize --force --fresh                                    \
  && make --jobs ${jobs} --keep-going --directory /opt/spack-environment \
     SPACK_ENV=${SPACK_ENV}                                              \