No worries I phrased that quite weird I think.
A NAS is only more power efficient if the additional power of a full server is not used. If for some reason the server is still needed than the NAS will be additional power consumption and not save anything.
(for example I run some quite RAM and compute heavy things on my server which no stock NAS could handle I think).
A Dockerfile itself is the instruction set. There is a certain minimum requirement expected from a server admin that differs from end-user requirements.
The ease of docker obfuscates that quite a bit but if you want to go full bare metal (or full AWS or GCS, etc etc) then you need to manage the full admin part as well - including custom deployments.