spindle/engines/nixery: extend container caps

kaniko (and other docker build tools) require permission to change file ownership to successfully build containers.

i'm leaving this here for consideration, not sure about this security wise,, the better solution would be to use kvm, i think. in our case docker is running in sysbox without `privileged: true` so it _should_ be fine(??)

Signed-off-by: zenfyr.dev <mail@melontini.me>