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

@@ -155,6 +155,7 @@ fi
 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} BUILDCACHE_MIRROR="local eics3rw"
 ccache --show-stats