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

@@ -129,7 +129,7 @@ EOF
 ## ========================================================================================
 FROM spack as builder
 
-## Setup our custom environment (secret mount for write-enabled mirror)
+## 1. Setup our default environment (secret mount for write-enabled mirror)
 COPY --from=spack-environment . /opt/spack-environment/
 ARG ENV=dev
 ENV SPACK_ENV=/opt/spack-environment/${ENV}
@@ -148,7 +148,7 @@ ccache --show-stats
 ccache --zero-stats
 EOF
 
-## Setup our custom environment with custom versions (on top of cached layer)
+## 2. Setup our environment with custom versions (on top of cached layer)
 ARG JUGGLER_VERSION=""
 ARG EICRECON_VERSION=""
 ADD https://eicweb.phy.anl.gov/api/v4/projects/EIC%2Fjuggler/repository/tree?ref=${JUGGLER_VERSION} /tmp/juggler.json