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.

containers/jug/dev.Dockerfile

@@ -59,7 +59,7 @@ RUN declare -A arch=(                                                   \
 ## Setup spack buildcache mirrors, including an internal
 ## spack mirror using the docker build cache, and
 ## a backup mirror on the internal B010 network
-RUN --mount=type=cache,target=/var/cache/spack-mirror,sharing=locked    \
+RUN --mount=type=cache,target=/var/cache/spack-mirror                   \
     export PATH=$PATH:$SPACK_ROOT/bin                                   \
  && spack mirror add docker /var/cache/spack-mirror                     \
  && spack buildcache update-index -d /var/cache/spack-mirror            \
@@ -68,7 +68,7 @@ RUN --mount=type=cache,target=/var/cache/spack-mirror,sharing=locked    \
 ## Setup eic-spack buildcache mirrors (FIXME: leaks credentials into layer)
 ARG S3_ACCESS_KEY=""
 ARG S3_SECRET_KEY=""
-RUN --mount=type=cache,target=/var/cache/spack-mirror,sharing=locked    \
+RUN --mount=type=cache,target=/var/cache/spack-mirror                   \
     export PATH=$PATH:$SPACK_ROOT/bin                                   \
  && if [ -n "${S3_ACCESS_KEY}" ] ; then                                 \
     spack mirror add --scope site                                       \