feat: install environment and push to buildcache with Makefile

This separates the installation step from the Dockerfile, allowing for extracting this into a separate repo or external phase.

spack-environment/Makefile

+MAKEFLAGS += -Orecurse
+
+SPACK ?= spack
+SPACK_INSTALL_FLAGS += --no-check-signature
+
+export SPACK_COLOR = always
+
+SPACK_ENV ?= dev
+
+BUILDCACHE_DIR := $(SPACK_ENV)/cache
+BUILDCACHE_MIRROR :=
+
+.PHONY: all clean
+
+all: $(SPACK_ENV)/push
+
+ifeq (,$(filter clean,$(MAKECMDGOALS)))
+include $(SPACK_ENV)/spack.mk
+endif
+
+$(SPACK_ENV)/push: $(addprefix $(SPACK_ENV)/push/,$($(SPACK_ENV)/SPACK_PACKAGE_IDS))
+	@mkdir -p $(BUILDCACHE_DIR)
+	$(foreach buildcache, $(BUILDCACHE_DIR), $(SPACK) buildcache update-index --mirror-url $(buildcache) ;)
+	$(foreach buildcache, $(BUILDCACHE_MIRROR), $(SPACK) buildcache update-index --mirror-url $(buildcache) ;)
+
+$(SPACK_ENV)/push/%: $(SPACK_ENV)/install/%
+	@mkdir -p $(dir $@)
+	$(foreach buildcache, $(BUILDCACHE_DIR), $(SPACK) buildcache create --allow-root --only=package --unsigned --directory $(buildcache) /$(HASH) ;) # push $(SPEC)
+	$(foreach buildcache, $(BUILDCACHE_MIRROR), $(SPACK) buildcache create --allow-root --only=package --unsigned --mirror-name $(buildcache) /$(HASH) ;) # push $(SPEC)
+	@touch $@
+
+$(SPACK_ENV)/spack.lock: $(SPACK_ENV)/spack.yaml Makefile
+	$(SPACK) concretize --force --fresh
+
+$(SPACK_ENV)/spack.mk: $(SPACK_ENV)/spack.lock Makefile
+	$(SPACK) env depfile --output $@ --make-target-prefix $(SPACK_ENV)
+
+clean:
+	rm -rf $(SPACK_ENV)/spack.lock $(SPACK_ENV)/spack.mk