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

@@ -102,13 +102,22 @@ RUN git clone --filter=tree:0 https://github.com/${EICSPACK_ORGREPO}.git ${EICSP
     fi                                                                  \
  && spack repo add --scope site "${EICSPACK_ROOT}"
 
+## Setup key4hep-spack
+ENV KEY4HEPSPACK_ROOT=${SPACK_ROOT}/var/spack/repos/key4hep-spack
+ARG KEY4HEPSPACK_ORGREPO="key4hep/key4hep-spack"
+ARG KEY4HEPSPACK_VERSION="main"
+ADD https://api.github.com/repos/${KEY4HEPSPACK_ORGREPO}/commits/${KEY4HEPSPACK_VERSION} /tmp/key4hep-spack.json
+RUN git clone --filter=tree:0 https://github.com/${KEY4HEPSPACK_ORGREPO}.git ${KEY4HEPSPACK_ROOT} \
+ && git -C ${KEY4HEPSPACK_ROOT} checkout ${KEY4HEPSPACK_VERSION}        \
+ && spack repo add --scope site "${KEY4HEPSPACK_ROOT}"
+
 ## ========================================================================================
 ## STAGE1: builder
 ## EIC builder image with spack environment
 ## ========================================================================================
 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/${ENV}
@@ -126,7 +135,7 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
  && ccache --show-stats                                                 \
  && ccache --zero-stats
 
-## 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=${JUGGLER_VERSION} /tmp/juggler.json
@@ -145,6 +154,7 @@ RUN --mount=type=cache,target=/ccache,id=${TARGETPLATFORM}              \
  && if [ -n ${EICRECON_VERSION} ] ; then                                \
       spack rm eicrecon && spack add eicrecon@git.${EICRECON_VERSION} ; \
     fi                                                                  \
+ && spack concretize --force --fresh                                    \
  && make --jobs ${jobs} --keep-going --directory /opt/spack-environment \
     SPACK_ENV=${SPACK_ENV}                                              \
     BUILDCACHE_MIRROR="local eics3rw"                                   \