ERROR: failed to solve: process "/bin/sh -c nix-env ...

I'm new to both Fly and NixOS so it could be the below is entirely my ignorance. Please illuminate if you can, thanks!

I opened this repo in Codespaces. Flyctl was not available in the terminal so I ran `docker build`, which bailed out with a failed to solve process error. Here's the output from a 2nd docker build run:

```
@maphew ➜ /workspaces/nix-builder (main) $ docker build .
[+] Building 35.1s (6/8) docker:default
=> [internal] load .dockerignore 0.1s
=> => transferring context: 2B 0.0s
=> [internal] load build definition from Dockerfile 0.1s
=> => transferring dockerfile: 172B 0.0s
=> [internal] load metadata for docker.io/nixos/nix:latest 0.4s
=> CACHED [1/4] FROM docker.io/nixos/nix@sha256:3f6c77ee4d2c82e472e64e6cd7087241dc391421 0.0s
=> [internal] load build context 0.1s
=> => transferring context: 59B 0.0s

=> ERROR [2/4] RUN nix-env -i mount curl rsync ps flyctl skopeo 38.5s
------
> [2/4] RUN nix-env -i mount curl rsync ps flyctl skopeo:
37.76 error: selector 'mount' matches no derivations, maybe you meant:
37.76 mount-util-linux
37.76 mount-zip
37.76 mountain-duck
------
Dockerfile:2
--------------------
1 | FROM nixos/nix
2 | >>> RUN nix-env -i mount curl rsync ps flyctl skopeo
3 | COPY boot.sh /boot.sh
4 | COPY rsyncd.conf /etc/rsyncd.conf
--------------------
ERROR: failed to solve: process "/bin/sh -c nix-env -i mount curl rsync ps flyctl skopeo" did not complete successfully: exit code: 1
```