spindle,shuttle,nix: add an alpine microvm image spindle/microvm: allow user defined binary caches in workflows shuttle,nix/microvm: get rid of the hacky nix config parsing / rendering, use nix directly so we can access module options spindle/engine: generalize scheduler out of microvm, make it work-conserving with aging and per-user fairness spindle/microvm: add resource budget limits and optional cgroup enforcement Signed-off-by: dawn <dawn@tangled.org>