feat: avoid large copy into runtime image

## This docker build follows two tracks, in order to ensure that we build all packages
## in a builder image, but install them in a runtime image, while at the same time
## avoiding a expensive filesystem copy operation at the end that breaks layering.
##
## The build is split in an infrequently-changing default environment, upon which
## an environment with custom versions (e.g. individual commits) is layered. The
## custom environment will change frequently but layers will be smaller, allowing
## for easier deployment with smaller delta layers.
##
## The separation in a builder and runtime image is particularly relevant to end up with
## lightweight images for expensive build dependencies, such as for example CUDA.
##
## builder track                         runtime track
## ----------------------------------------------------------------------
## builder_image                         runtime_image
## builder_concretization_default   
## builder_installation_default     ->   runtime_concretization_default  (copy spack.lock)
##                                 \->   runtime_installation_default    (from buildcache)
## builder_concretization_custom
## builder_installation_custom      ->   runtime_concretization_custom   (copy spack.lock)
##                                 \->   runtime_installation_custom     (from buildcache)