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

@@ -149,10 +149,10 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
  && spack buildcache update-index eics3rw                               \
  && spack env activate --dir ${SPACK_ENV}                               \
  && if [ -n ${JUGGLER_VERSION} ] ; then                                 \
-      spack config add "packages:juggler:require:['@git.${JUGGLER_VERSION}']" \
+      spack config add "packages:juggler:require:['@git.${JUGGLER_VERSION}']" ; \
     fi                                                                  \
  && if [ -n ${EICRECON_VERSION} ] ; then                                \
-      spack config add "packages:eicrecon:require:['@git.${EICRECON_VERSION}']" \
+      spack config add "packages:eicrecon:require:['@git.${EICRECON_VERSION}']" ; \
     fi                                                                  \
  && spack concretize --force --fresh                                    \
  && make --jobs ${jobs} --keep-going --directory /opt/spack-environment \